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ENV330 Module 2A AVP Transcript
Title Slide
Narrator: Let’s review some basic ecology. Levels of organization of nature, population dynamics,
ecosystems, ecological succession, and biogeochemical cycles.
Slide 2
Title: Levels of Organization of Nature
Slide Content: [an image depicting the levels of organization]
Narrator: Ecology is the study of the interdependencies among living organisms and between organisms
and their physical, that is, non-living, environment. Nature is made up of a hierarchy of nested systems,
each self-organizing and self-maintaining.
All organisms are made up of molecules which are made up of atoms. All matter, including the rocky
Earth, the oceans and the atmosphere, are made up of atoms. The complex molecules that make up
living things are also made up of atoms.
Those complex molecules, such as proteins, carbohydrates, fats and DNA, make up the structure of the
cells of which all organisms are comprised. All living things are made up of cells – the cell is the simplest
level of organization of nature which is alive, that is, which is self-organizing, self-maintaining, able to
reproduce itself, senses its environment and responds to it, takes in high quality energy and materials and
releases low quality energy and wastes.
In multicellular organisms, cells are organized into tissues. Different tissues form layers which make up
organs. Different organs together form organ systems. Each organism is made up of many integrated
organ systems.
Groups of organisms of the same species which live in the same place are called populations.
Populations of organisms of different types which live together in one place are called biological
communities.
Ecosystems are self-organizing, self-maintaining groups of interdependent plants, animals, micro-
organisms and their physical environment, through which energy flows and materials recycle. A lake
ecosystem would include all the populations of fish, plankton, larger plants, the mud on the bottom — with
its myriad creatures including soil bacteria, protozoans, worms, mussels, larval insects – the water in the
lake, the sunlight entering the lake, the heat leaving the lake, the mineral nutrients in the mud and all the
water, etc., etc.
Ecosystems can make up huge sections of continents called biomes defined by similar terrains and
climates. In North America, the Temperate Deciduous Forest Biome extends from the Atlantic Ocean to
the Mississippi and from the Canada to the Gulf of Mexico.
All of the biomes of Earth – all of the living systems of Earth – taken together, make up the Ecosphere or
Biosphere. The Ecosphere operates as if it is a single system with complex feedback loops. The
Ecosphere of Earth, is like a thin, moist membrane on the rocky crust of the Earth and including the
oceans and lower atmosphere, it is almost as if it is alive! The very atmosphere of the Earth is the joint
exhalations of all of the life on Earth! This is not true on any other planet that we have observed. It is a
living blue-green jewel in the vast abyss of space.
Slide 3
Title: Ecosystems
Slide content:
[image of a drop of water on a leaf]
Narrator: Ecosystems are self-organizing, self-maintaining groups of interdependent plants, animals and
microorganisms – and their physical environment, dirt, air, water, light, etc. – through which energy flows,
and materials recycle.
High quality energy, sunlight, enters and powers the ecosystem, but is degraded due to the 2
nd
Law of
Energy into low quality “waste” heat energy. (The 2
nd
law of energy or thermodynamics, remember, states
that whenever energy is converted from one form to another, the total energy quality must decline).
Living organisms must have a dependable supply of high quality energy in order to stay alive. Plants use
high quality, concentrated sunlight energy, and animals and decomposers use high quality concentrated
chemical energy (food). Plants convert the high quality sunlight energy into high quality chemical energy
(sugar and other complex organic molecules). Animals eat plants or other animals that have eaten
plants.
The matter of an ecosystem is recycled around and around and around. The molecules that make up
your body have been part of dinosaurs, trees, rocks, the air, the water — and will be parts of other
creatures and the air and the rocks again. With each breath we exhale CO2 which will be taken in by a
plant and used to make their body. And, with each breath we take in O2 released by a plant to be used by
us to break down the sugars that we’ve taken in, by eating plants and animals, into energy that powers
our bodies.
We are connected at a deep and basic level to all the plants, animals and microorganisms of Earth – and
to the rocks and air and water!
Slide 4
Title: Food Chains and The Pyramid of Energy
Slide Content:
[image of a leaf, grasshopper, mice, a snake and an owl]
Narrator: A food chain is the linear flow of energy and materials through one pathway of a food web.
Each link in a food chain is called a trophic level. Heat is “lost” from the food chain at each step due to
the 2
nd
Law of Energy. The energy conversions that take place in every cell of every organism have a
cost – “there ain’t no such thing as a free lunch”! That is, every time energy is converted from one form to
another, as happens each second in every living cell, the energy quality always must decline. High
quality concentrated chemical energy stored in the body as sugar or starch is converted to low quality
“waste” heat energy which leaves the body and therefore is unavailable to creatures further along the
food chain. You can’t eat heat!
Question: is energy lost from the universe in this process?
No. The laws of conservation of mass and energy tell us that the total amount of energy – and matter – in
the universe is constant. Energy and matter can be converted from one form to another but can neither
be created nor destroyed. That’s the law!
The Pyramid of Energy: The total amount of biologically useable energy declines by about 90% at each
step in a food chain – because of the 2
nd
Law of Energy. Therefore, food chains are never more than
about 4-5 links long because so little useable energy is left as you move along it. At each step in a food
chain most of the energy is converted into low quality “waste” heat energy, which is lost to the food chain.
So, in a food chain comprised of sunlight, grass, grasshoppers, mice, snakes and hawks, there will be
very few hawks, not very many snakes, quite a few mice, and lots of grasshoppers!
Question: Why can more people be fed if they subsist on a vegetarian diet?
Answer: Because the food chain is shorter and so less energy is lost as waste heat due to the 2
nd
Law.
Livestock convert about 90% of the grain fed to them into waste heat energy which is lost to the food
chain. If people are fed the grain directly, skipping the livestock, 10-20 times as many people can be fed!
Slide 5
Title: Food Webs
Slide Content:
[image of sea lions on the beach]
Narrator: Food webs are much more complex than food chains. They involve dozens or hundreds or
even thousands of interconnections – each organism eating many different kinds of food and each in term
being preyed upon by several different predators. Think about coral reef ecosystems or tropical rain
forest ecosystems. In such complex, interdependent systems, disruptions of any part of the system
effects all parts of the system and can result in surprising consequences.
For example, let’s consider the Antarctic food web, where all sorts of creatures migrate to take advantage
of the huge and rapid growth of plankton during the Antarctic summer. If lots of Killer Whales are
successfully hunted by humans, fewer Leopard Seals would be eaten by Killer Whales, so their numbers
would go up and they would eat more Crabeater Seals, Elephant Seals and Squid, reducing their
numbers. Emperor Penguins would be dramatically affected since one of their main sources of food is
squid. They would eat more fish, allowing the zooplankton – tiny microscopic animals — to increase in
numbers, perhaps clouding the water and cutting down on the amount of light that the microscopic
phytoplankton – plants at the base of the food web – receive. Those effects would ripple through the
entire ecosystem.
Slide 6
Title: Ecological Succession
Slide Content:
[image of a field of wheat]
Narrator: Ecosystems normally undergo successive stages of development over time, one ecological
community replacing another. Each stage changes the physical and ecological environment, thus
creating conditions that allow the next stage to develop. Each Biome has characteristic stages of
ecological succession.
Primary succession starts from bare rock with Pioneer Species such as lichens and mosses, and
progresses ultimately to the Climax stage of development such as a mature hardwood forest. It can take
centuries or thousands of years for a system to go through all the stages. The early stages from bare
rock to lichens and mosses, for example, take the most time and are the most fragile.
Secondary Ecological Succession starts with soil and some plants and other organisms already present,
such as an abandoned farmer’s field. It may take centuries for the land to restore itself. But, Nature is
resilient — if a system is not too severely degraded it can recover over time.
Succession is a back and forth process. Disturbances such as storms, fires, floods, Wallmart parking
lots, halt the successional process, pushing the ecosystem back to a previous stage. Large ecosystems
will be spotty with different sections at different stages of succession due to their histories of fire, storm or
other disturbances. This spottiness actually increases the biological diversity of the larger system,
making it healthier. Some animals and plants thrive in the transitional boundaries between areas at
different stages – the ecotones, as these transitional areas are termed. Thus, within a forest there may
be meadows of grass where a fire took out trees years ago. Deer and other creatures move back and
forth between the two habitats.
Slide 7
Title: Population Dynamics
Slide Content:
[image of a timeline of population sizes]
Narrator: Ecosystems are comprised of populations of organisms of particular species. Populations go
through stages as they develop. At first, a new population will grow exponentially while resources are not
limiting – its rate of increase increasing, its doubling time shortening. The ability of populations to grow so
rapidly is called their Biotic Potential.
Each ecosystem has a set Carrying Capacity for each population, the point at which the natural resources
that the species depends upon becomes limiting. This could be food, water, space, waste processing
ability of the micro-organisms in the soil, or many other factors. The carrying capacity is the maximum
number of individuals which can be sustained without degrading the ecosystem.
Following the initial period of rapid exponential growth, there is a period of adjustment to the realities of
the natural world – the logistic phase. In this phase, populations come into balance with the carrying
capacity. If they overshoot the carrying capacity, thus degrading the environment, their numbers go down
if and until the rest of the environment recovers and then their numbers may increase again to stable
levels.
Sometimes populations so exceed their carrying capacities, and cause so much ecosystem degradation
that they completely crash to zero or to very low numbers. For example, when 26 reindeer (24 of them
female) were introduced to a small Bering Sea island of St. Paul in 1910, lichens, mosses, and other food
sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying
capacity. This led to a population crash, and the herd size plummeted to only 8 reindeer by 1950.
Populations sometimes never recover if the damage is bad enough.
How about the human population? Current UN median world population projections, assuming that by
2050 women will have 2.0 children per lifetime, is that we will reach 9.3 billion people by 2050. The world
population is currently over 6.5 billion.
Questions:
What kind of growth has the human population experienced in the last 50 years?
What is the Carrying Capacity for Humans on Earth?
Are there any signs that we have exceeded it?
Is there any environmental degradation or resource depletion evident caused by the human population?
What will happen to the human population if it has drastically exceeded the Earth’s carrying capacity for
humans?
Answers: The carrying capacity of Earth for humans depends on our lifestyles. An Indian peasant farmer
uses far less resources and creates far less waste than you or I do. They have a very small ecological
footprint whereas you and I have rather large ecological footprints. This concept is known as the Cultural
Carrying Capacity. The Earth can support far more people living simply than it can people living high
consumption, high natural capital degradation lifestyles.
It has been calculated that a population of perhaps 40 billion peasants eating a bowl of rice a day could
be supported on an Earth totally cleared of all ecosystems and devoted solely to growing grain. Raise
your hand if you volunteer for that lifestyle!
Is there evidence that humans have overshot their carrying capacity? Plenty – degraded air, water, soil,
ecosystems, climate, oceans — the ecological footprint of humans today is far greater than the planet’s
resources.
What could happen? Human populations could crash just like any other organism that exceeds its
carrying capacity. Many past civilizations have declined and disappeared due to resource abuse and
degradation of their natural environment.
In this course you will learn about ways to stabilize the human population and avoid a human population
crash, and create sustainable societies.
Slide 8
Title: Predator-Prey Relationship
Slide Content:
[image of a lion]
Narrator: Populations within an ecosystem which directly depend upon each other affect each other
dramatically. When there are many lions hunting zebra, the zebra population declines, providing less
food for the lions, and so the lion population declines. With less lions, less zebra are hunted and so the
zebra population increases providing more food for the lions and thus allowing the lion population to
increase. Thus their populations directly affect one another.
In addition to this predator-prey relationship both populations are also affected by other populations of
organisms in their food webs. Everything is interconnected, or as Naturalist John Muir said almost a
hundred years ago, “When we try to pick out anything by itself, we find it hitched to everything else in the
universe”.
End of Presentation
ENV330 Module 2b AVP Transcript
Slide 1
Title: Biogeochemical Cycles
Narrator: Everything on the Earth, in the air, in the water, and below the surface of the Earth is recycled
in great cycles involving Life, Rocks, Air, Water and the molecules and atoms which make them up.
The familiar water or hydrologic cycle is a good example of these cycles. Most of the world’s water is
found in the oceans which cover almost ¾ the of the planet with an average depth of 2 ½ miles. Radiant
energy from the sun – sunlight – is converted into heat energy when it interacts with the surface of the
Earth, including water. The heat energy causes some of the water molecules to evaporate and form
water vapor; clouds, which move around the Earth with wind currents.
When precipitation occurs in the form of rain, snow, hail, etc., the water vapor is converted back into liquid
or solid water (ice) and gravity pulls it back down to the surface of the Earth where it either stays frozen
as part of glaciers in high mountains, or as part of the frozen polar ice caps, or is absorbed as liquid water
into the land and rivers and lakes.
Plants absorb some of the water and animals drink some and eat the plants and so it enters into the food
webs and ecosystems. All of the water is pulled by gravity across the land towards the lowest places,
forming rivers and eventually flowing to the sea. Some of the water trickles down through layers of soil
and rock to become ground water – aquifers, which also move slowly underground toward the oceans.
Water is also evaporated from plants, rivers and lakes back into the atmosphere.
Note that this cycle has been affected, as have all the biogeochemical cycles, by human activities. Some
of the impacts include: Reduced recharge of land and flooding from covering land with buildings and
crops, increased flooding from wetlands destruction, point source pollution of surface waters by industrial
discharges, non-point source runoff of pollutants into waterways, aquifer depletion from over-pumping of
wells, and, dramatic changes in rainfall patterns and melting of ice caps and glaciers due to global
climate change.
Another biogeochemical cycle, the carbon cycle is critical to all of life on Earth, since C is the atomic
backbone of all the organic molecules which make up the cells of all living things. It is also critically
important in its role related to Global Climate Change due to human activities.
Most of Earth’s C is found as CO2 in the atmosphere. Plants take up the CO2 and use the C to build their
bodies. This is called the “fixing” of C. Animals eat the plants and each other and decomposers eat both,
thus spreading the carbon – the basis of all organic molecules that make up life – throughout all
ecosystems. When animals and decomposers exhale they release CO2 back into the atmosphere.
Some C is stored deep in the rocks as coal, natural gas and oil. This C was part of the partially
decomposed organic matter from productive ecosystems like marshes and swamps where layers and
layers of leaves and other biological debris form rich, deep black muck and peat which over the millions of
years, if covered by other sediments, can become converted into “fossil fuels” – ancient stored sunlight!
The photosynthesis of plants converts light energy into chemical energy. Coal, natural gas and oil are the
ancient chemical energy made by ancient plants using ancient sunlight.
CO2 is also dissolved into the oceans where ecosystems use it just as on the land. In addition, many
marine organisms absorb C dissolved in the oceans as CaCO3 (calcium carbonate) to make their shells
and other hard parts. The shells of clams and oysters, and many of the myriad plankton are made of
CaCO3 , as are the skeletons of coral organisms. When these organisms die, especially the plankton,
their limestone shells accumulate on the seafloor by the billions and over geological time become
cemented together to form limestone rock layers miles deep. Tectonic movement of crustal plates can
result in the uplifting of this limestone rock which can then become part of the continents where they may
be eroded by surf and wind and rivers, thus releasing the C back into the waters and soil where it may be
taken up by plants once again.
Some CO2 is released to the atmosphere from volcanic activity, as has always been the case on Earth.
There are many harmful impacts of human activities on the Carbon Cycle which have led to a dramatic
increase in the atmospheric CO2 concentrations, which is the main cause of Global Climate Change.
Starting with agriculture some 10-12,000 years ago, humans began to burn down forests in order to plant
crops. The combustion of wood converts the tons of C in organic molecules that make up the trees into
CO2. During the industrial revolution starting in the 18
th
century, millions of tons of coal were burned to
power the factories and mills and steam engines. Coal is a fossil fuel made up of C. The combustion of
coal results in the release of tons of CO2 into the atmosphere. Oil was discovered in Pennsylvania in
1859. Billions of barrels of oil have been combusted since resulting in a catastrophic increase in
atmospheric CO2 levels. The concentration of CO2 in the atmosphere today is dramatically higher than it
has been on Earth for the last 900,000 years.
Other important biogeochemical cycles include the N cycle, the P cycle and the S cycle.
The Nitrogen cycle is also critical for life on Earth. Without N there would be no protein or enzymes.
Without proteins and enzymes there would be no life. Most of the N on Earth is in the atmosphere. Soil
bacteria play a critical role in making this cycle function. Human activities have also caused many
disruption of this cycle, including fertilizer runoff into waterways resulting in fish kills.
The Phosphorous cycle is additionally critical to life. Without P the energy transformation reactions that
take place in every cell would be impossible. Most of the P on Earth is in certain sedimentary rocks
associated with bird guano! Note again, that many disruptions of this cycle are caused by human
activities.
Sulfur is another important component of living organisms. The sulfur cycle is also degraded and altered
by human activities which cause acid rain deposition leading to deforestation and the acidification of
lakes.
All the biogeochemical cycles are interconnected with each other and with the rock cycle and atmospheric
and oceanic circulation. All of these critical biogeochemical cycles are being disrupted by human
activities.
In this course we will learn how to interact with these important cycles in a more responsible and
sustainable way.
End of Presentation
Compose
a 300
–
word (minimum) essay on the topic below. Essays must be double
–
spaced
and use APA
–
style in
–
text citations to reference ideas or quotes that are not your own. You
must include a separate bibliography.
Can the world provide an adequate standa
rd of living for a projected 2.6 billion more
people by the year 2050 without causing widespread environmental damage? Explain.
You should cite and quote from assigned readings, AVP’s, videos, and module activities to
support the ideas in your essay.
Compose a 300
–
word (minimum) essay on the topic below. Essays must be double
–
spaced
and use APA
–
style in
–
text citations to reference ideas or quotes that are not your own. You
must include a separate bibliography.
Can the world provide an adequate standa
rd of living for a projected 2.6 billion more
people by the year 2050 without causing widespread environmental damage? Explain.
You should cite and quote from assigned readings, AVP’s, videos, and module activities to
support the ideas in your essay.
Compose a 300-word (minimum) essay on the topic below. Essays must be double-spaced
and use APA-style in-text citations to reference ideas or quotes that are not your own. You
must include a separate bibliography.
Can the world provide an adequate standard of living for a projected 2.6 billion more
people by the year 2050 without causing widespread environmental damage? Explain.
You should cite and quote from assigned readings, AVP’s, videos, and module activities to
support the ideas in your essay.
Perspective
Are
humans part of nature? Are humans part of ecosystems?
Consider holding your breathe for 10 minutes. What would happen?
What do you eat? What do you drink? What do you breathe? Which animals and plants do
you eat? How do they grow? Where does the matter and energy come from that sustains
them? Where does the water that you drink come from? What was that water part of before
you drank it? Where was it before that? Where does the air come from that you breathe?
How was it made? Who or what exhaled it? What inhales your exhalation?
What happens to animal and plant “waste” and dead? What would happen if humans
disappeared from
Earth? What would happen if decomposers disappeared from Earth?
For this module, you are required to read the following from your textbook:
·
Chapter 3
·
Chapter 4
·
Chapter 5
·
Chapter 6
To help guide and focus your reading, the textbook includes a list of
Key Questions
at the
beginning of each chapter, and a
Review of Key Concepts
at the end of the chapter.
You are advised to review these before and after reading the chapters, as this will help you
prepare for the course assignments, discussions, and exams.
The Habitable Planet: A Systems Approach to Environmental Science
View
Episode 4
–
Ecosystems
Ecosystems Video
–
Annenberg Learner
Ecosystems Video – Annenberg Learner
1. Click here to access the video series website
2. Click “Individual Program Descriptions”
3. Click the VoD link to the right of the assigned program title to launch the video
Questions to consider while viewing video:
· Why do ecosystems like tropical rainforests have such immense diversity?
· What have scientists discovered that determines how many individuals of a species can be supported within an ecosystem?
· How does science restore the diversity to areas where human activity has interfered with the natural structure of a habit/ecosystem?
The Habitable Planet: A Systems Approach to Environmental Science
View Episode 5 – Human Population Dynamics
Perspective
Are humans part of nature? Are humans part of ecosystems?
Consider holding your breathe for 10 minutes. What would happen?
What do you eat? What do you drink? What do you breathe? Which animals and plants do
you eat? How do they grow? Where does the matter and energy come from that sustains
them? Where does the water that you drink come from? What was that water part of before
you drank it? Where was it before that? Where does the air come from that you breathe?
How was it made? Who or what exhaled it? What inhales your exhalation?
What happens to animal and plant “waste” and dead? What would happen if humans
disappeared from
Earth? What would happen if decomposers disappeared from Earth?
For this module, you are required to read the following from your textbook:
·
Chapter 3
·
Chapter 4
·
Chapter 5
·
Chapter 6
To help guide and focus your reading, the textbook includes a list of
Key Questions
at the
beginning of each chapter, and a
Review of Key Concepts
at the end of the chapter.
You are advised to review these before and after reading the chapters, as this will help you
prepare for the course assignments, discussions, and exams.
The Habitable Planet: A Systems Approach to Environmental Science
View
Episode 4
–
Ecosystems
Ecosystems Video
–
Annenberg Learner
Perspective
Are humans part of nature? Are humans part of ecosystems?
Consider holding your breathe for 10 minutes. What would happen?
What do you eat? What do you drink? What do you breathe? Which animals and plants do
you eat? How do they grow? Where does the matter and energy come from that sustains
them? Where does the water that you drink come from? What was that water part of before
you drank it? Where was it before that? Where does the air come from that you breathe?
How was it made? Who or what exhaled it? What inhales your exhalation?
What happens to animal and plant “waste” and dead? What would happen if humans
disappeared from Earth? What would happen if decomposers disappeared from Earth?
For this module, you are required to read the following from your textbook:
Chapter 3
Chapter 4
Chapter 5
Chapter 6
To help guide and focus your reading, the textbook includes a list of Key Questions at the
beginning of each chapter, and a Review of Key Concepts at the end of the chapter.
You are advised to review these before and after reading the chapters, as this will help you
prepare for the course assignments, discussions, and exams.
The Habitable Planet: A Systems Approach to Environmental Science
View Episode 4 – Ecosystems
Ecosystems Video – Annenberg Learner
Core
Case Study
Why Are Amphibians Vanishing?
Learning Objective
· LO 4.1List three reasons why we need to care about the growing rate of amphibian extinctions.
Amphibians are a class of animals that includes frogs (chapter
opening photo
, toads, and salamanders. Amphibians were among the first vertebrates (animals with backbones) to leave the earth’s waters and live on land. They have adjusted to and survived environmental changes more effectively than many other species, but their environment is changing rapidly.
An amphibian lives part of its life in water and part on land. Human activities such as the use of pesticides and other chemicals can pollute the land and water habitats of amphibians. Many of the more than 8,000 known amphibian species (90% of them frogs) have problems adapting to these changes.
Since 1980, populations of hundreds of amphibian species have declined or vanished (
Figure 4.1
). According to the International Union for Conservation of Nature (IUCN), about 33% of known amphibian species face extinction. A 2015 study by biodiversity expert Peter Crane found that 200 frog species have gone extinct since the 1970s, and frogs are going extinct 10,000 times faster than their historical rates.
Figure 4.1
Specimens of some of the nearly 200 amphibian species that have gone extinct since the 1970s.
Joel Sartore/National Geographic Image Collection
No single cause can account for the decline of many amphibian species, but scientists have identified a number of factors that affect amphibians at various points in their life cycles. For example, frog eggs lack shells to protect the embryos they contain from water pollutants and adult frogs ingest the insecticides contained in many of the insects they eat. We explore these and other factors later in this chapter.
Why should we care if some amphibian species become extinct? Scientists give three reasons. First, amphibians are sensitive biological indicators of changes in environmental conditions. These changes include habitat loss, air and water pollution, ultraviolet (UV) radiation from the sun, and a warming climate. The growing threats to the survival of an increasing number of amphibian species indicate that environmental conditions for amphibians and many other species are deteriorating in many parts of the world.
Second, adult amphibians play important roles in biological communities. They eat more insects (including mosquitoes) than do many species of birds. In some habitats, the extinction of certain amphibian species could lead to population declines or extinction of animals that eat amphibians or their larvae, such as reptiles, birds, fish, mammals, and other amphibians.
Third, amphibians play a role in human health. A number of pharmaceutical products come from compounds found in secretions from the skin of certain amphibians. Many of these compounds have been isolated and used as painkillers and antibiotics and in treatments for burns and heart disease. If amphibians vanish, these potential medical benefits and others that scientists have not yet discovered would vanish with them.
The threat to amphibians is part of a greater threat to the earth’s biodiversity. In this chapter, we discuss biodiversity, how it arose on the earth, why it is important, and how it is threatened. We will also consider possible solutions to these threats.
4.1aEarth’s Organisms Are Many and Varied
Every organism is composed of one or more cells. Based on their cell structure, organisms can be classified as eukaryotic or prokaryotic. All organisms except bacteria are
eukaryotic
. Their cells are encased in a membrane and have a distinct nucleus (a membrane-bounded structure containing genetic material in the form of DNA) and several other internal parts enclosed by membranes (
Figure 4.2
, right). Bacterial cells are
prokaryotic
, enclosed by a membrane but containing no distinct nucleus or other internal parts enclosed by membranes (Figure 4.2, left).
Figure 4.2
Comparison of key components of a eukaryotic cell (left) and prokaryotic cell (right).
Scientists group organisms into categories based on their varying characteristics, a process called taxonomic classification. The largest category is the kingdom, which includes all organisms that have one or several common features. Biologists recognize six kingdoms. Two are different types of bacteria (eubacteria and archaebacteria) with single cells that are prokaryotic (Figure 4.2, left). The other four kingdoms are protists, plants, fungi, and animals (Figure 4.2 right).
Protists are mostly many-celled eukaryotic organisms such as golden brown and yellow-green algae, and protozoans. Most fungi are many-celled organisms such as mushroom, molds, mildews, and yeasts.
Plants include certain types of algae (including red, brown, and green algae), mosses, ferns, trees, and flowering plants whose flowers produce seeds. Flowering plant species make up about 90% of the plant kingdom. Some flowering plants such as corn and marigolds are
annuals
that live for one growing season, die, and have to be replanted. Others are
perennials
, such as roses and grapes, which can live for two or more seasons before they die and have to be replanted.
Animals are many-celled eukaryotic organisms. Most are
invertebrates
, with no backbones. They include jellyfish, worms, insects, shrimp, snails, clams, and octopuses. Other animals, called
vertebrates
, have backbones. Examples include amphibians (
Core Case Study
), fishes, reptiles (alligators and snakes), birds (robins and eagles), and mammals (humans, whales, elephants, bats, and tigers).
Kingdoms are divided into phyla, which are divided into subgroups called classes. Classes are subdivided into orders, which are further divided into families. Families consist of genera (singular, genus), and each genus contains one or more species.
Figure 4.3
shows the detailed taxonomic classification for the current human species: Homo sapiens sapiens.
Figure 4.3
How the current human species got its name: Homo sapiens sapiens.
A species is a group of living organisms with characteristics that distinguish it from other groups of organisms. In sexually reproducing organisms, individuals must be able to mate with similar individuals and produce fertile offspring in order to be classified as a species.
Estimates of the number of species range from 7 million to 100 million, with a best guess of 7 million to 10 million species. Biologists have identified about 2 million species.
2 Million
Number of species that scientists have identified out of the world’s estimated 7 million to 100 million species
Well over half of the world’s identified species are insects that play important ecological roles in sustaining the earth’s life. For example, pollination is a vital ecosystem service that allows flowering plants to reproduce. When pollen grains are transferred from the flower of one plant to a receptive part of the flower of another plant of the same species, reproduction occurs. Many flowering species depend on bees and other insects to pollinate their flowers (
Figure 4.4
, left). In addition, insects that eat other insects—such as the praying mantis (Figure 4.4, right)—help to control the populations of at least half the species of insects that we call pests. This free pest control service is another vital ecosystem service. In addition, insects make up an increasing part of the human food supply in some parts of the world.
Figure 4.4
Importance of insects: Bees (left) and numerous other insects pollinate flowering plants that serve as food for many plant eaters, including humans. This praying mantis, which is eating a moth (right), and many other insect species help to control the populations of most of the insect species we classify as pests.
Klagyivik Viktor/ Shutterstock.com; Dr. Morley Read/ Shutterstock.com
Some insect species reproduce at an astounding rate and can rapidly develop new genetic traits such as resistance to pesticides. They also have an exceptional ability to evolve into new species when faced with changing environmental conditions.
Research indicates that some human activities are threatening insect populations such as honeybees. We discuss this environmental problem more fully in
Chapter 9
.
Learning from Nature
Even the lowly mosquito provides benefits to humans by serving as a model for a new type of hypodermic needle, based on the mosquito’s use of a multi-part mouth to work its way through a layer of skin without creating pain. (The pain of a mosquito bite comes from a chemical injected into the skin after it has been penetrated.)
4.2aBiodiversity
Biodiversity, or
biological diversity
, is the variety of life on the earth. It has four components, as shown in
Figure 4.5
.
Figure 4.5
Natural capital: The major components of the earth’s biodiversity—one of the planet’s most important renewable resources and a key component of its natural capital (
Figure 1.3
).
Right side, top left: Laborant/ Shutterstock.com; right side, top right: leungchopan/ Shutterstock.com; right side, top center: Elenamiv/ Shutterstock.com; bottom right: Juriah Mosin/ Shutterstock.com.
One is
species diversity
, the number and abundance of the different kinds of species living in an ecosystem. Species diversity has two components, one being
species richness
, the number of different species in an ecosystem. The other is
species evenness
, a measure of the comparative abundance of all species in an ecosystem.
A species-rich ecosystem has a large number of different species. However, this tells us nothing about how many members of each species are present. If it has many of one or more species and just a few of others, its species evenness is low. If it has roughly equal numbers of each species, its species evenness is high. For example, if an ecosystem has only three species, its species richness is low. However, if there are roughly equal numbers of each of the three species, the species evenness is high. Species-rich ecosystems such as rain forests tend to have high species evenness. Ecosystems with low species richness, such as tree farms, tend to have low species evenness.
Species diversity can enhance the stability of ecosystems. For example, a forest with many different tree species is more stable than a forest with just one tree species, which is the case with a tree farm.
The species diversity of ecosystems varies with their geographical location. For most terrestrial plants and animals, species diversity (primarily species richness) is highest in the tropics and declines as we move from the equator toward the poles. The most species-rich environments are tropical rain forests, large tropical lakes, coral reefs, and the ocean-bottom zone.
The second component of biodiversity is
genetic diversity
, which is the variety of genes found in a population or in a species (
Figure 4.6
). Genes contain genetic information that give rise to specific traits, or characteristics, that are passed on to offspring through reproduction. Species whose populations have greater genetic diversity have a better chance of surviving and adapting to environmental changes.
Figure 4.6
Genetic diversity in this population of a Caribbean snail species is reflected in the variations of shell color and banding patterns. Genetic diversity can also include other variations such as slight differences in chemical makeup, sensitivity to various chemicals, and behavior.
The third component of biodiversity,
ecosystem diversity
, refers to the earth’s diversity of biological communities such as deserts, grasslands, forests, mountains, oceans, lakes, rivers, and wetlands. Biologists classify terrestrial (land) ecosystems into
biomes
—large regions such as forests, deserts, and grasslands characterized by distinct climates and certain prominent species (especially vegetation). Biomes differ in their community structure based on the types, relative sizes, and stratification of their plant species (
Figure 4.7
).
Figure 4.8
shows the major biomes found across the midsection of the United States. We discuss biomes in detail in
Chapter 7
.
Figure 4.7
Community structure: Generalized types, relative sizes, and stratification of plant species in communities or ecosystems in major terrestrial biomes.
Figure 4.8
The variety of biomes found across the midsection of the United States.
First: Zack Frank/ Shutterstock.com; second: Robert Crum/ Shutterstock.com; third: Joe Belanger/ Shutterstock.com; fourth: Protasov AN/ Shutterstock.com; fifth: Maya Kruchankova/ Shutterstock.com; sixth: Marc von Hacht/ Shutterstock.com
Large areas of forest and other biomes tend to have a core habitat and edge habitats with different environmental conditions and species, called
edge effects
. For example, a forest edge is usually more open, bright, and windy and has greater variations in temperature and humidity than a forest interior. Humans have fragmented many forests, grasslands, and other biomes into isolated patches with less core habitat and more edge habitat that supports fewer species.
Natural ecosystems within biomes rarely have distinct boundaries. Instead, one ecosystem tends to merge with the next in a transitional zone called an
ecotone
. It is a region containing a mixture of species from adjacent ecosystems along with some migrant species not found in either of the bordering ecosystems.
The fourth component of biodiversity is
functional diversity
—the variety of processes such as energy flow and matter cycling that occur within ecosystems (
Figure 3.9
) as species interact with one another in food chains and food webs. This component of biodiversity includes the variety of ecological roles organisms play in their biological communities and the impacts these roles have on their overall ecosystems.
A more biologically diverse ecosystem with a greater variety of producers can produce more plant biomass, which in turn can support a greater variety of consumer species. Biologically diverse ecosystems also tend to be more stable because they are more likely to include species with traits that enable them to adapt to changes in the environment, such as disease or drought.
We should care about and avoid degrading the earth’s biodiversity because it is vital to maintaining the natural capital (
Figure 1.3) that keeps us alive and supports our economies. We use biodiversity as a source of food, medicine, building materials, and fuel. Biodiversity also provides natural ecosystem services such as air and water purification, renewal of topsoil, decomposition of wastes, and pollination. In addition, the earth’s variety of genetic information, species, and ecosystems provide raw materials for the evolution of new species and ecosystem services, as they respond to changing environmental conditions. Biodiversity is an ecological life insurance policy. When we celebrate, protect, and enhance the earth’s biodiversity, we are helping to preserve our own species and economic systems, which depend on the natural capital that biodiversity provides. We owe much of what we know about biodiversity to researchers such Edward O. Wilson (
Individuals Matter 4.1
).
Individuals Matter 4.1
Edward O. Wilson: A Champion of Biodiversity
Jim Harrison
As a boy growing up in the southeastern United States, Edward O. Wilson became interested in insects at age 9. He has said, “Every kid has a bug period. I never grew out of mine.”
Before entering college, Wilson had decided he would specialize in the study of ants. He became one of the world’s experts on ants and then widened his focus to include the entire biosphere. One of Wilson’s landmark works is The Diversity of Life, published in 1992. In that book, he presented the principles and practical issues of biodiversity more completely than anyone had to that point. Today, he is recognized as one of the world’s leading experts on biodiversity—often referred to as “the father of biodiversity.”
Wilson continues actively writing and lecturing about the importance of species and the need for global biodiversity discovery and inventory in order to better understand our planet and identify conservation priorities. In 2016, he published Half-Earth: Our Planet’s Fight for Life, a call to conserve half the Earth’s lands and seas in order to ensure species have the space they need to thrive in perpetuity. The E. O. Wilson Biodiversity Foundation is now bringing this vision to life through the Half-Earth Project.
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4.3aEach Species Plays a Role
Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as a species’
ecological niche
. It is a species’ way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. A species’ niche differs from its
habitat
, which is the place, or type of ecosystem, in which a species lives and obtains what it needs to survive.
Ecologists use the niches of species to classify them as generalists or specialists. A
generalist species
such as a raccoon has a broad niche (
Figure 4.9
, right curve). Generalist species can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species are flies, cockroaches, rats, coyotes, and humans.
Figure 4.9
Specialist species such as the giant panda have a narrow niche (left curve) and generalist species such as the raccoon have a broad niche (right curve).
In contrast, a
specialist species
, such as the giant panda, occupies a narrow niche (
Figure 4.9
, left curve). Such species may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of some shorebirds feed on certain crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands (
Figure 4.10
).
Figure 4.10
Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources.
Because of their narrow niches, specialists are more likely to become endangered or extinct when environmental conditions change. For example, China’s giant panda (
Figure 4.9
, left) is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mainly of bamboo.
Is it better to be a generalist or a specialist? It depends. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist species usually is better off.
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4.3aEach Species Plays a Role
Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as a species’
ecological niche
. It is a species’ way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. A species’ niche differs from its
habitat
, which is the place, or type of ecosystem, in which a species lives and obtains what it needs to survive.
Ecologists use the niches of species to classify them as generalists or specialists. A
generalist species
such as a raccoon has a broad niche (
Figure 4.9
, right curve). Generalist species can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species are flies, cockroaches, rats, coyotes, and humans.
Figure 4.9
Specialist species such as the giant panda have a narrow niche (left curve) and generalist species such as the raccoon have a broad niche (right curve).
In contrast, a
specialist species
, such as the giant panda, occupies a narrow niche (
Figure 4.9
, left curve). Such species may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of some shorebirds feed on certain crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands (
Figure 4.10
).
Figure 4.10
Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources.
Because of their narrow niches, specialists are more likely to become endangered or extinct when environmental conditions change. For example, China’s giant panda (
Figure 4.9
, left) is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mainly of bamboo.
Is it better to be a generalist or a specialist? It depends. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist species usually is better off.
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4.3cIndicator Species
Species that provide early warnings of changes in environmental conditions in an ecosystem are called
indicator species
. They are like biological smoke alarms. In this chapter’s Core Case Study, you learned that some amphibians are classified as indicator species. Scientists have been working hard to identify some of the possible causes of the declines in amphibian populations (
Science Focus 4.1
).
Science Focus 4.1
Causes of Amphibian Declines
Scientists who study amphibians have identified natural and human-related factors that can cause the decline and disappearance of these indicator species.
One natural threat is parasites such as flatworms that feed on certain amphibian eggs. Research indicates that this has caused birth defects such as missing limbs or extra limbs in some amphibians.
Another natural threat comes from viral and fungal diseases. An example is the chytrid fungus that infects a frog’s skin and causes it to thicken. This reduces the frog’s ability to take in water through its skin and leads to death from dehydration. An even deadlier fungal disease is Batrachochytrium dendrobatidis (or Bd), which invades skin cells and multiplies, causing the frog’s skin to peel away. Such diseases can spread easily, because adults of many amphibian species congregate in large numbers to breed.
Habitat loss and fragmentation is another major threat to amphibians. It is mostly a human-caused problem resulting from the clearing of forests and the draining and filling of freshwater wetlands for farming and urban development.
Another human-related problem is higher levels of UV radiation from the sun. Ozone that forms in the stratosphere protects the earth’s life from harmful UV radiation emitted by the sun. During the past few decades, ozone-depleting chemicals released into the troposphere by human activities have drifted into the stratosphere and have destroyed some of the stratosphere’s protective ozone. The resulting increase in UV radiation can kill embryos of amphibians in shallow ponds as well as adult amphibians basking in the sun for warmth. International action has been taken to reduce the threat of stratospheric ozone depletion, but it will take about 50 years for ozone levels to recover to levels that existed before this threat arose.
Pollution from human activities also threatens amphibians. Frogs and other species are exposed to pesticides in ponds and in the bodies of insects that they eat. This can make them more vulnerable to bacterial, viral, and fungal diseases and to some parasites. Amphibian expert and National Geographic Explorer Tyrone Hayes, a professor of biology at University of California-Berkeley, conducts research on how some pesticides can harm frogs and other animals by disrupting their endocrine systems.
Climate change is also a concern. Amphibians are sensitive to even slight changes in temperature and moisture. Warmer temperatures may lead amphibians to breed too early. Extended dry periods also lead to a decline in amphibian populations by drying up breeding pools that frogs and other amphibians depend on for reproduction and survival through their early stages of life (
Figure 4.A
).
Figure 4.A
This golden toad lived in Costa Rica’s high-altitude Monteverde Cloud Forest Reserve. The species became extinct in 1989, apparently because its habitat dried up.
Charles H. Smith/U.S. Fish and Wildlife Service
Overhunting is another human-related threat, especially in areas of Asia and Europe, where frogs are hunted for their leg meat. Another threat is the invasion of amphibian habitats by nonnative predators and competitors, such as certain fish species. Some of this immigration into habitats is natural, but humans accidentally or deliberately transport many species to amphibian habitats.
According to most amphibian experts, a combination of these factors, which vary from place to place, is responsible for most of the decline and extinctions of amphibian species. This amounts to a biological “fire alarm.”
Critical Thinking
Critical Thinking
1. Of the factors listed above, which three do you think could be most effectively controlled by human efforts?
Birds are excellent biological indicators. They are found almost everywhere and are affected quickly by environmental changes such as the loss or fragmentation of their habitats and the introduction of chemical pesticides.
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4.3dKeystone Species
A keystone is the wedge-shaped stone placed at the top of a stone archway. Remove this stone and the arch collapses. In some communities and ecosystems, ecologists hypothesize that certain species play a similar role. A
keystone species
has such a large effect on the types and abundance of other species in an ecosystem that without it, the ecosystem would be dramatically different or might cease to exist.
Keystone species play several critical roles in helping to sustain ecosystems. One is the pollination of flowering plant species by butterflies, honeybees (Figure 4.4, left), hummingbirds, bats, and other species. In addition, top predator keystone species feed on and help to regulate the populations of other species. Examples are wolves, leopards, lions, some shark species, and the American alligator (see the following Case Study).
Case Study
The American Alligator—A Keystone Species That Almost Went Extinct
The American alligator (
Figure 4.12
) is a keystone species in wetland ecosystems where it is found in the southeastern United States. These alligators play several important ecological roles. They dig deep depressions, or gator holes. These depressions hold freshwater during dry spells and serve as refuges for aquatic life. They supply freshwater and food for fishes, insects, snakes, turtles, birds, and other animals.
Figure 4.12
Keystone species: The American alligator plays an important ecological role in its marsh and swamp habitats in the southeastern United States by helping support many other species.
Arto Hakola/ Shutterstock.com
The large nesting mounds that alligators build provide nesting and feeding sites for some herons and egrets, and red-bellied turtles lay their eggs in old gator nests. In addition, by eating large numbers of gar, a predatory fish, alligators help maintain populations of game fish that gar eat, such as bass and bream. When alligators excavate holes and build nesting mounds, they help keep vegetation from invading shorelines and open-water areas. Without this ecosystem service, freshwater ponds and coastal wetlands where alligators live would fill in with shrubs and trees, and dozens of species could disappear from these ecosystems.
In the 1930s, hunters began killing American alligators for their exotic meat and their soft belly skin, used to make expensive shoes, belts, and pocketbooks. Other people hunted alligators for sport or out of dislike for the large reptile. By the 1960s, hunters and poachers had wiped out 90% of the alligators in the state of Louisiana, and the Florida Everglades population waswas near extinction.
In 1967, the U.S. government placed the American alligator on the endangered species list. By 1987, because it was protected, its populations had made a strong comeback and the alligator was removed from the endangered species list. Today, there are well over a million alligators in Florida. The state now allows property owners to kill alligators that stray onto their land.
To conservation biologists, the comeback of the American alligator is an important success story in wildlife conservation. Recently, however, large and rapidly reproducing Burmese and African pythons released deliberately or accidently by humans have invaded the Florida Everglades. These nonnative invaders feed on young alligators, and could threaten the long-term survival of this keystone species in the Everglades.
Critical Thinking
The American Alligator and Biodiversity
1. What are two ways in which the American alligator supports one or more of the four components of biodiversity (
Figure 4.5) within its environment?
The loss of a keystone species in an ecosystem can lead to population declines and, in some cases, to extinctions of other species that depend on them for certain ecosystem services. This is why it important for scientists to identify keystone species and work to protect them.
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4.4aEvolution Explains How Life Changes Over Time
How did the earth end up with such an amazing diversity of species? The scientific answer is
biological evolution
or simply
evolution
—the process by which the genes of populations of species change genetically over time. According to this scientific theory, species have evolved from earlier, ancestral species through
natural selection
—the process in which individuals with certain genetic traits are more likely to survive and reproduce under a specific set of environmental conditions. These individuals then pass these traits on to their offspring.
A huge body of scientific evidence supports this idea. As a result, biological evolution through natural selection is the most widely accepted scientific theory that explains how the earth’s life has changed over the past 3.8 billion years and why we have today’s diversity of species.
Most of what we know about the history of life on the earth comes from
fossils
—the remains or traces of past organisms. Fossils include mineralized or petrified replicas of skeletons, bones, teeth, shells, leaves, and seeds, or impressions of such items found in rocks (
Figure 4.13
). Scientists have discovered fossil evidence in successive layers of sedimentary rock such as limestone and sandstone. They have also studied evidence of ancient life contained in ice core samples drilled from glacial ice at the earth’s poles and on mountaintops.
Figure 4.13
This fossil shows the mineralized remains of an early ancestor of the present-day horse. It roamed the earth more than 35 million years ago. Notice that you can also see fish skeletons on this fossil.
Ira Block/National Geographic Image Collection
This body of evidence is called the fossil record. It is uneven and incomplete because many past forms of life left no fossils and some fossils have decomposed. Scientists estimate that the fossils found so far represent probably only 1% of all species that have ever lived. There are still many unanswered scientific questions about the details of evolution by natural selection, and research continues in this area.
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4.4bEvolution Depends on Genetic Variability and Natural Selection
The idea that organisms change over time and are descended from a single common ancestor has been discussed since the early Greek philosophers. There was no convincing explanation of how this could happen until 1858 when naturalists Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) independently proposed the concept of natural selection as a mechanism for biological evolution. Darwin gathered evidence for this idea and published it in his 1859 book, On the Origin of Species by Means of Natural Selection.
Biological evolution by natural selection involves changes in a population’s genetic makeup through successive generations. Populations—not individuals—evolve by becoming genetically different.
The first step in this process is the development of
genetic variability
: a variety in the genetic makeup of individuals in a population. This occurs primarily through
mutations
, or changes in the coded genetic instructions in the DNA in a gene. During an organism’s lifetime, the DNA in its cells is copied each time one of its cells divides and whenever it reproduces. In a lifetime, this happens millions of times and results in various mutations.
Most mutations result from random changes in the DNA’s coded genetic instructions that occur in only a tiny fraction of these millions of divisions. Some mutations also occur from exposure to external agents such as radioactivity, ultraviolet (UV) radiation from the sun, and certain natural and human-made chemicals called mutagens.
Mutations can occur in any cell, but only those that take place in the genes of reproductive cells are passed on to offspring. Sometimes a mutation can result in a new genetic trait, called a heritable trait, which can be passed from one generation to the next. In this way, populations develop genetic differences among their individuals. Biologists refer to the genes of a population as a
gene pool
.
The next step in biological evolution is natural selection, which explains how populations evolve in response to changes in environmental conditions by changing their genetic makeup. Through natural selection, environmental conditions favor increased survival and reproduction of certain individuals in a population. These favored individuals possess heritable traits that give them an advantage over other individuals in the population. Such a trait is called an
adaptation
, or
adaptive trait
. An adaptive trait improves the ability of an individual organism to survive and to reproduce at a higher rate than other individuals in a population can under prevailing environmental conditions.
An example of natural selection at work is genetic resistance. It occurs when one or more organisms in a population have genes that can tolerate a chemical (such as a pesticide or antibiotic) that normally would be fatal. The resistant individuals survive and reproduce more rapidly than the members of the population that do not have such genetic traits. Genetic resistance can develop quickly in populations of organisms such as bacteria and insects that produce large numbers of offspring. For example, some disease-causing bacteria have developed genetic resistance to widely used antibacterial drugs, or antibiotics (
Figure 4.14
).
Figure 4.14
Evolution by natural selection: (a) A population of bacteria is exposed to an antibiotic, which (b) kills all individuals except those possessing a trait that makes them resistant to the drug; (c) the resistant bacteria multiply and eventually (d) replace all or most of the nonresistant bacteria.
Through natural selection, humans have evolved traits that have enabled them to survive in many different environments and to reproduce successfully. If we think of the earth’s 4.6 billion years of geological and biological history as one 24-hour day, the human species arrived about a tenth of one second before midnight. In that short time, we have dominated most of the earth’s land (
Figure 1.9
) and aquatic systems. Evolutionary biologists attribute our ability to dominate the earth to three major adaptations (
Figure 4.15
):
· Strong opposable thumbs that allowed humans to grip and use tools better than the few other animals that have thumbs
· The ability to walk upright, which gave humans agility and freed up their hands for many uses
· A complex brain, which allowed humans to develop many skills, including the ability to communicate complex ideas.
Figure 4.15
Homo sapiens sapiens: Three advantages over other mammals have helped us to become the earth’s dominant species within an eye blink of time in the 3.8-billion-year history of life on the earth.
To summarize the process of biological evolution by natural selection: Genes mutate, individuals are selected, and populations that are better adapted to survive and reproduce under existing environmental conditions evolve.
Evolutionary biologists study patterns of evolution by examining the similarities and differences among species based on their physical and genetic characteristics. They use this information to develop branching
evolutionary tree
, or
phylogenetic tree
, diagrams that depict the hypothetical evolution of various species from common ancestors (
Figure 4.16
). They use fossil, DNA, and other evidence to hypothesize the evolutionary pathways and connections among species.
Figure 4.16
Simplified phylogenetic tree (or tree of life) diagram showing the hypothesized evolution of life on the earth into six major kingdoms of species over 3.8 billion years.
On an evolutionary timescale, as new species arise, they have new genetic traits that can enhance their survival as long as environmental conditions do not change dramatically. The older species from which they originated and the new species evolve and branch out along different lines or lineages of species that can be recorded in phylogenetic tree diagrams (
Figure 4.16
).
4.4cLimits to Adaptation through Natural Selection
In the not-too-distant future, will adaptations to new environmental conditions through natural selection protect us from harm? For example, will adaptations allow the skin of our descendants to become more resistant to the harmful effects of the sun’s UV radiation, enable their lungs to cope with air pollutants, and improve the ability of our livers to detoxify pollutants in our bodies?
Scientists in this field say not likely because of two limitations on adaptation through natural selection. First, a change in environmental conditions leads to adaptation only for genetic traits already present in a population’s gene pool, or if they arise from random mutations.
Second, even if a beneficial heritable trait is present in a population, the population’s ability to adapt may be limited by its reproductive capacity. Populations of genetically diverse species that reproduce quickly often can adapt to a change in environmental conditions in a short time (days to years). Examples are dandelions, mosquitoes, rats, bacteria, and cockroaches. By contrast, species that cannot produce large numbers of offspring rapidly—such as elephants, tigers, sharks, and humans—take thousands or even millions of years to adapt through natural selection.
4.4dMyths about Evolution through Natural Selection
There are a number of misconceptions about biological evolution through natural selection. Here are five common myths:
· Survival of the fittest means survival of the strongest. To biologists, fitness is a measure of reproductive success, not strength. Thus, the fittest individuals are those that leave the most descendants, not those that are physically the strongest.
· Evolution explains the origin of life. It does not. However, it does explain how species have evolved after life came into being around 3.8 billion years ago.
· Humans evolved from apes or monkeys. Fossil and other evidence shows that humans, apes, and monkeys evolved along different paths from a common ancestor that lived 5 million to 8 million years ago.
· Evolution by natural selection is part of a grand plan in nature in which species are to become more perfectly adapted. There is no evidence of such a plan. Instead, evidence indicates that the forces of natural selection and random mutations can push evolution along any number of paths.
· Evolution by natural selection is not important because it is just a theory. This reveals a misunderstanding of the concept of a scientific theory, which is based on extensive evidence and is accepted widely by the scientific experts in a particular field of study. Numerous polls show that evolution by natural selection is widely accepted by over 95% of biologists because it best explains the earth’s biodiversity and how populations of different species have adapted to changes in the earth’s environmental conditions over billions of years.
4.5aHow Do New Species Arise?
Under certain circumstances, natural selection can lead to an entirely new species. Through this process, called
speciation
, one species evolves into two or more different species. For sexually reproducing organisms, a new species forms when a separated population of a species evolves to the point where its members can no longer interbreed and produce fertile offspring with members of another population of its species that did not change or that evolved differently.
Speciation, especially among sexually reproducing species, happens in two phases: first geographic isolation, and then reproductive isolation.
Geographic isolation
occurs when different groups of the same population of a species become physically isolated from one another for a long time. Part of a population may migrate in search of food and then begin living as a separate population in an area with different environmental conditions. Winds and flowing water may carry a few individuals far away where they establish a new population. A flooding stream, a new road, a hurricane, an earthquake, or a volcanic eruption, as well as long-term geological processes (
Science Focus 4.2
), can also separate populations. Human activities, such as the creation of large reservoirs behind dams and the clearing of forests, can also create physical barriers for certain species. The separated populations can develop different genetic characteristics because they are no longer exchanging genes.
Science Focus 4.2
Geological Processes Affect Biodiversity
The earth’s surface has changed dramatically over its long history. Scientists discovered that huge flows of molten rock within the earth’s interior have broken its surface into a number of gigantic solid plates, called tectonic plates. For hundreds of millions of years, these plates have drifted slowly on the planet’s mantle formed today’s continents (
Figure 4.B
).
Figure 4.B
Over millions of years, the earth’s landmasses have moved very slowly on several gigantic tectonic plates to different locations and formed today’s continents (right).
Critical Thinking:
1. How might an area of land splitting apart cause the extinction of a species?
Rock and fossil evidence indicates that 200–250 million years ago, all of the earth’s present-day continents were connected in a supercontinent called Pangaea (Figure 4.B, left). About 175 million years ago, Pangaea began splitting apart as the earth’s tectonic plates moved. Eventually tectonic movement resulted in the present-day locations of the continents (Figure 4.B, right).
The movement of tectonic plates has had two important effects on the evolution and distribution of life on the earth. First, the locations of continents and oceanic basins have greatly influenced the earth’s climate, which plays a key role in where plants and animals can live. Second, the breakup, movement, and joining of continents have allowed species to move and adapt to new environments. This led to the formation of a large number of new species through speciation.
Along boundaries where they meet, tectonic plates may pull away from, collide with, or slide alongside each other. Tremendous forces produced by these interactions along plate boundaries can lead to earthquakes and volcanic eruptions. These geological activities can also affect biological evolution by causing fissures in the earth’s crust, which can isolate populations of species on either side of the fissure. Over long periods, this can lead to the formation of new species as each isolated population changes genetically in response to new environmental conditions.
Volcanic eruptions that occur along the boundaries of tectonic plates can also affect extinction and speciation by destroying habitats and reducing, isolating, or wiping out populations of species. These geological processes are further discussed in
Chapter 14
.
Critical Thinking
1. The earth’s tectonic plates, including the one you are riding on, typically move at about the rate at which your fingernails grow. If they stopped moving, how might this affect the future biodiversity of the planet?
In
reproductive isolation
, mutation and change by natural selection operate independently in the gene pools of geographically isolated populations. If this process continues long enough, members of isolated populations of sexually reproducing species can become different in genetic makeup. Then they cannot produce live, fertile offspring if they rejoin their original populations and attempt to interbreed. When that is the case, speciation has occurred and one species has become two (
Figure 4.17
).
Figure 4.17
Geographic isolation can lead to reproductive isolation, divergence of gene pools, and speciation.
4.5bArtificial Selection, Genetic Engineering, Gene Editing, and Synthetic Biology
For thousands of years, humans have used
artificial selection
to change the genetic characteristics of populations with similar genes. First, they select one or more desirable genetic traits that already exist in the population of a plant or animal. Then they use selective breeding, or crossbreeding, to control which members of a population have the opportunity to reproduce to increase the numbers of individuals in a population with the desired traits (
Figure 4.18
).
Figure 4.18
Artificial selection involves the crossbreeding of species that are close to one another genetically. In this example, similar fruits are being crossbred to yield a pear with a certain color.
Learning from Nature
Artificial selection is a classic case of learning from nature. It involves learning how natural processes produce a particular trait in a fruit or vegetable and then using crossbreeding to enhance that trait.
Artificial selection is not a form of speciation. It is limited to crossbreeding between genetic varieties of the same species or between species that are genetically similar to one another. Most of the grains, fruits, and vegetables we eat are produced by artificial selection. Artificial selection has also given us food crops with higher yields, cows that give more milk, trees that grow faster, and many different varieties of dogs and cats. However, traditional crossbreeding is a slow process.
Scientists have learned how to speed this process of manipulating genes in order to get desirable genetic traits or eliminate undesirable ones. They do this by transferring segments of DNA with the desired trait from one species to another through a process called
genetic engineering
. In this process, also known as gene splicing, scientists alter an organism’s genetic material by adding, deleting, or changing segments of its DNA to produce desirable traits or to eliminate undesirable ones. Scientists have used genetic engineering to develop modified crop plants, new drugs, pest-resistant plants, and animals that grow rapidly.
There are five steps in this process:
1. Identify a gene with the desired trait in the DNA found in the nucleus of a cell from the donor organism.
2. Extract a small circular DNA molecule, called a plasmid, from a bacterial cell.
3. Insert the desired gene from
step 1
into the plasmid to form a recombinant DNA plasmid.
4. Insert the recombinant DNA plasmid into a cell of another bacterium, which rapidly divides and reproduces large numbers of bacterial cells with the desired DNA trait.
5. Transfer the genetically modified bacterial cells to a plant or animal that is to be genetically modified.
The result is a
genetically modified organism (GMO)
with its genetic information modified in a way not found in natural organisms. Genetic engineering enables scientists to transfer genes between different species that would not interbreed in nature. For example, scientists can put genes from a cold-water fish species into a tomato plant to give it properties that help it resist cold weather. Recently, scientists have learned how to treat certain genetic diseases by altering or replacing the genes that cause them. Genetic engineering has revolutionized agriculture and medicine. However, it is a controversial technology, as we discuss in
Chapter 12
.
In 2012, scientists developed a new gene editing technique called RISPRCRISPR. This easy and cheap technique allows researchers to snip, insert, delete, or modify genetic material at targeted spots in DNA molecules with increased precision.
Gene editing has great promise for correcting disease-causing mutations in DNA molecules. It could be used treat cancers and other human diseases and to modify the DNA in human embryos to remove disease-causing mutations. It has been used to cure mice with HIV and hemophilia and to engineer pigs to make them suitable organ donors for humans. Gene editing can be done to cells outside the human body. Then the modified cells can be implanted in the human body. These genetic changes can then be passed on to future generations.
One worry is that gene editing could become so easy and cheap (a gene editing kit can be ordered online for $150) that anybody could do it and not be subject to regulations. Individuals could possibly alter cells in human embryos, eggs, and sperm to come up with “designer babies” with genes that favor certain traits. This could lead to discrimination against certain groups and a host of other major ethical challenges.
A new and rapidly growing form of genetic engineering is synthetic biology. It enables scientists to make new sequences of DNA and use such genetic information to design and create artificial cells, tissues, body parts, and organisms not found in nature. Synthetic biology can bypass the long process of evolution by natural selection and create new forms of life in a short time.
This process starts with a computer code of an organism’s entire genetic sequence (genome). Then engineers insert new sequences of the four nucleotide bases, adenine (A), cytosine (C), guanine (G), and thymine (T) (
Figure 2.9
), to create a new and different genetic sequence, or genome. Next, they transplant the new genome into the cell of a bacterium to transform it into a different, human-created species of bacteria. This technology uses science and engineering to alter the planet’s life by reducing the cell to a machine that can assemble forms of life like products in a factory.
Proponents of synthetic biology want to use it to create bacteria that can use sunlight to produce clean-burning hydrogen gas, which can be used to fuel motor vehicles. This could help us reduce our dependence on fossil fuels. Synthetic biology might also be used to create bacteria and algae that break down oil, industrial wastes, toxic heavy metals, pesticides, and radioactive waste in contaminated soil and water. It could be used to create vaccines to prevent diseases and drugs to combat parasitic diseases such as malaria. It might be used to develop instructions for three-dimensional printers to print human body parts, car parts, and clothing.
Scientists are a long way from achieving such goals, but the potential is there. If used properly and ethically, this new technology could help us live more sustainably. The problem is that, like any technology, synthetic biology can be used for good or bad. For example, it could be used to create biological weapons such as deadly bacteria that spread new diseases, to destroy existing oil deposits, or to interfere with the chemical cycles that keep us alive. It might also end up hindering the ability of decomposers to breakdown and recycle wastes, or it might add new pollutants to soil and water. This is why many scientists call for increased monitoring and regulation of this new technology to help control its use.
Learning from Nature
Scientists are applying synthetic biology by studying how organisms in nature operate. For example, some bacteria can consume substances that are harmful to humans, and scientists hope to create a bacterium that can be used to cleanse the human body of such substances.
4.5cExtinction Eliminates Species
Another factor affecting the number and types of species on the earth is
biological extinction
, or simply extinction, which occurs when an entire species ceases to exist. When environmental conditions change dramatically, a population of a species faces three possible futures: adapt to the new conditions through natural selection, migrate (if possible) to another area with more favorable conditions, or become extinct in the area where they are found.
Species found in only one area, called
endemic species
, are especially vulnerable to extinction. They exist on islands and in other isolated areas. For example, many species in tropical rain forests have highly specialized roles and are vulnerable to extinction. These organisms are unlikely to be able to migrate or adapt to rapidly changing environmental conditions. Many of these endangered species are amphibians (Core Case Study).
Extinction is a natural and ongoing process. Fossils and other scientific evidence indicate that 99.9% of all species that have existed on the earth are now extinct. Throughout most of the earth’s long history, species have disappeared at a low rate, called the
background extinction rate
. Based on the fossil record and analysis of ice cores, biologists estimate that the average annual background extinction rate has been about 0.0001% of all species per year, which amounts to 1 species lost for every million species on the earth per year. At this rate, if there were 10 million species on the earth, about 10 of them, on average, would go extinct every year.
Evidence indicates that life on the earth has been sharply reduced by several periods of
mass extinction
during which extinction rates rise well above the background rate. In such a catastrophic, widespread, and often global event, 50-95% of all species are wiped out because of major, widespread environmental changes such as long-term climate change, massive flooding because of rising sea levels, huge meteorites striking the earth’s surface, and gigantic volcanic eruptions. Such events can trigger drastic environmental changes on a global scale, including massive releases of debris into the atmosphere that block sunlight for an extended period. This can kill off most plant species and the consumers that depend on them for food. Fossil and geological evidence indicates that there have been five mass extinctions (at intervals of 25–60 million years) during the past 500 million years (
Figure 4.19
).
Figure 4.19
Scientific evidence indicates that the earth has experienced five mass extinctions over the past 500 million years and that human activities have initiated a new sixth mass extinction.
A mass extinction provides an opportunity for the evolution of new species that can fill unoccupied ecological niches or newly created ones. Scientific evidence indicates that each past mass extinction has been followed by an increase in species diversity as shown by the wedges in Figure 4.19). However, this recovery process takes millions of years.
As environmental conditions change, the balance between speciation and extinction determines the earth’s biodiversity. The existence of millions of species today means that speciation, on average, has kept ahead of extinction. However, evidence indicates that the global extinction rate is rising sharply. Many scientists see this is as evidence that we are experiencing the beginning of a new sixth mass extinction caused mostly by human activities (Figure 4.19). We examine this issue and ways to deal with it in Chapter 9. The
Case Study
that follows discusses the threat of extinction for the monarch butterfly because of human activities.
Case Study
The Threatened Monarch Butterfly
The beautiful North American monarch butterfly (
Figure 4.20
and the front cover of this book) is in trouble. This species is known for its annual 3,200- to 4,800-kilometer (2,000- to 3,000-mile) migration from the northern United States and Canada to a small number of tropical forest areas in central Mexico. They arrive on a predictable schedule and later return to their North American home. Another monarch population in the Midwestern United States makes a shorter annual journey to coastal northern California and then returns home.
Figure 4.20
Monarch butterflies in Mexico.
Melinda Fawer/ Shutterstock.com
During their annual round-trip journeys, these two populations of monarchs face serious threats from bad weather and numerous predators. In 2002, a single winter storm killed an estimated 75% of the monarch population migrating to Mexico.
During their migration, the monarchs need access to milkweed plants to lay their eggs. Once the butterfly larvae hatch, the caterpillar survives to become a butterfly by feeding on the milkweed plant. Without milkweed, the monarch butterfly cannot reproduce and faces extinction.
Once the monarchs reach their winter forest destinations in Mexico and California, they cling to trees (
Figure 4.20) by the millions as they rest. Each year, biologists estimate the monarch’s population size by measuring the total areas of forest they occupy at these destinations.
The overall estimated monarch population varies from year to year, mostly because of changes in weather and other natural conditions. However, the U.S. Fish and Wildlife Service estimates that since 1975, this overall population has dropped by nearly a billion. The size of Monarch butterfly populations wintering in Mexican forests varies from year to year, often because of weather and other environmental factors. However, since 1996, there has been an overall decline in their annual populations.
The monarchs face three serious threats from human activities in addition to the historic natural threats. One threat is the steady loss of their winter forest habitat in Mexico, due to logging (most of it illegal), and loss of their northern California habitat due to coastal development. A second threat is reduced access to milkweed plants essential for their survival during their migration. Almost all of the natural prairies in the United States, which were abundant with milkweed plants, have been replaced by croplands where milkweed plants grow much more sparsely only as weeds between rows of crops and on roadsides.
A third threat over the past decade is the explosive growth of cropland in the American Midwest planted with corn and soybean varieties genetically engineered to resist herbicides that are used to kill weeds, including milkweed. Some of these herbicides are thought to be killing monarchs as well as their food source.
So why should we care if the monarch butterfly becomes extinct, largely because of human activities? One reason is that monarchs provide an important ecological service by pollinating a variety of flowering plants (including corn) along their migration routes as they feed on the nectar from the blossoms of such plants. Another reason for many people is the belief that it is ethically wrong for us to cause the premature extinction of the monarch butterfly or other species.
What can we do to reduce the threat to this amazing species? Researchers call for protecting their migratory pathways and for the government to protect the monarch by classifying it as a threatened species. They propose that we sharply reduce the use of herbicides to kill milkweed. In addition, many people are trying to help by planting milkweed and other plants that attract pollinators such as butterflies and honeybees (whose populations are also decreasing, as we discuss in Chapter 9).
Learning from Nature
Scientists are studying the Monarch butterfly to find out how they are able to navigate their age-old annual migration routes and arrive at the same places in Mexico and California on the same day of each year. This knowledge could have benefits for human aviation.
Big Ideas
· Each species plays a specific ecological role, called its niche, in the ecosystems where it is found.
· As environmental conditions change, the genes in some individuals mutate and give those individuals genetic traits that enhance their abilities to survive and to produce offspring with these traits.
· The degree of balance between speciation and extinction in response to changing environmental conditions determines the earth’s biodiversity, which helps to sustain the earth’s life and our economies.
Tying It All TogetherAmphibians and Sustainability
Robert King/ Shutterstock.com
This chapter’s
Core Case Study describes the increasing losses of amphibian species and explains why these species are important ecologically. In this chapter, we studied the importance of biodiversity—the numbers and varieties of species found in different parts of the world, along with genetic, ecosystem, and functional diversity.
We examined the variety of niches, or roles played by species in ecosystems. For example, we saw that some species, including many amphibians, are indicator species that warn us about threats to biodiversity, ecosystems, and the biosphere. Others such as the American alligator are keystone species that play vital roles in sustaining the ecosystems where they live.
We also studied the scientific theory of biological evolution through natural selection, which explains how life on the earth changes over time due to changes in the genes of populations and how new species can arise. We learned that the earth’s species biodiversity is the result of a balance between the formation of new species (speciation) and extinction of species due to changing environmental conditions.
The ecosystems where amphibians and other species live are functioning examples of the three scientific principles of sustainability in action. These species depend on solar energy, the cycling of nutrients, and biodiversity. Disruptions in any of these forms of natural capital can result in degradation of these species’ populations and their ecosystems.
Chapter Review
Critical Thinking
1. What might happen to humans and a number of other species if most or all amphibian species (Core Case Study) were to go extinct?
2. How might a reduction in species diversity affect the other three components of biodiversity?
3. Is the human species a keystone species? Explain. If humans become extinct, what are three species that might also become extinct and what are three species whose populations might grow?
4. Why should we care about saving the monarch butterfly from extinction caused by human activities? Do you care? Why or why not?
5. How would you respond to someone who tells you:
1. We should not believe in biological evolution because it is “just a theory.”
2. We should not worry about air pollution because natural selection will enable humans to develop lungs that can detoxify pollutants.
6. How would you respond to someone who says that because extinction is a natural process, we should not worry about the loss of biodiversity when species become extinct largely because of our activities?
7.
List three aspects of your lifestyle that could be contributing to some of the losses of the earth’s biodiversity. For each of these, what are some ways to avoid making this contribution?
8. Congratulations! You are in charge of the future evolution of life on the earth. What are the three most important things that you would do? Explain.
Chapter Review
Doing Environmental Science
1. Study an ecosystem of your choice, such as a meadow, a patch of forest, a garden, or an area of wetland. (If you cannot do this physically, do so virtually by reading about an ecosystem online or in a library.) Determine and list five major plant species and five major animal species in your ecosystem. Which, if any, of these species are indicator species and which of them, if any, are keystone species? Explain how you arrived at these hypotheses. Then design an experiment to test each of your hypotheses, assuming you would have unlimited means to carry them out.
Chapter Review
Data Analysis
The following table is a sample of a very large body of data reported by J. P. Collins, M. L. Crump, and T. E. Lovejoy III in their book Extinction in Our Times—Global Amphibian Decline. It compares various areas of the world in terms of the number of amphibian species found and the number of amphibian species that were endemic, or unique to each area. Scientists like to know these percentages because endemic species tend to be more vulnerable to extinction than do non-endemic species. Study the table and then answer the following questions.
Area
Number of Species
Number of Endemic Species
Percentage Endemic
Pacific/Cascades/Sierra Nevada Mountains of North America
52
43
Southern Appalachian Mountains of the United States
101
37
Southern Coastal Plain of the United States
68
27
Southern Sierra Madre of Mexico
118
74
Highlands of Western Central America
126
70
Highlands of Costa Rica and Western Panama
133
68
Tropical Southern Andes Mountains of Bolivia and Peru
132
101
Upper Amazon Basin of Southern Peru
102
22
1. Fill in the fourth column by calculating the percentage of amphibian species that are endemic to each area.
2. Which two areas have the highest numbers of endemic species? Name the two areas with the highest percentages of endemic species.
3. Which two areas have the lowest numbers of endemic species? Which two areas have the lowest percentages of endemic species?
4. Which two areas have the highest percentages of non-endemic species?
Core Case Study
Why Are Amphibians
Vanishing?
Learning Objective
·
LO 4.1
List three reasons why we need to care about the growing rate of amphibian
extinctions.
Amphibians are a class of animals that includes frogs (chapter
opening
photo
, toads, and salamanders. Amphibians were among the first
vertebrates (animals with backbones) to leave the earth’s waters and
live on land. They have adjusted to and survived environmental changes
more effectively
than many other species, but their environment is
changing rapidly.
An amphibian lives part of its life in water and part on land. Human
activities such as the use of pesticides and other chemicals can pollute
the land and water habitats of amphibians. Man
y of the more than 8,000
known amphibian species (90% of them frogs) have problems adapting
to these changes.
Since 1980, populations of hundreds of amphibian species have declined
or vanished (
Figure 4.1
). According to the In
ternational Union for
Conservation of Nature (IUCN), about 33% of known amphibian species
face
extinction
. A 2015 study by biodiversity expert Peter Crane found
that 200 frog species have gone extinct since the 1970s, and frogs are
going extinct 10,000 tim
es faster than their historical rates.
Figure
4.1
Specimens of some of the nearly 200 amphibian species that have gone
extinct since the 1970s.
Core Case StudyWhy Are Amphibians
Vanishing?
Learning Objective
LO 4.1List three reasons why we need to care about the growing rate of amphibian
extinctions.
Amphibians are a class of animals that includes frogs (chapter opening
photo, toads, and salamanders. Amphibians were among the first
vertebrates (animals with backbones) to leave the earth’s waters and
live on land. They have adjusted to and survived environmental changes
more effectively than many other species, but their environment is
changing rapidly.
An amphibian lives part of its life in water and part on land. Human
activities such as the use of pesticides and other chemicals can pollute
the land and water habitats of amphibians. Many of the more than 8,000
known amphibian species (90% of them frogs) have problems adapting
to these changes.
Since 1980, populations of hundreds of amphibian species have declined
or vanished (Figure 4.1). According to the International Union for
Conservation of Nature (IUCN), about 33% of known amphibian species
face extinction. A 2015 study by biodiversity expert Peter Crane found
that 200 frog species have gone extinct since the 1970s, and frogs are
going extinct 10,000 times faster than their historical rates.
Figure 4.1
Specimens of some of the nearly 200 amphibian species that have gone
extinct since the 1970s.
Main
content
Chapter Introduction
A clownfish gains protection by living among sea anemones and
help
s
protect the anemones from some of their
predators.
cbpix/
Shutterstock.com
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Core
Case Study
The Southern Sea
Otter: A Species in Recovery
Learning Objective
·
LO 5.1
Explain what could happen to the Pacific coast kelp
forest
ecosystem if the southern sea otters were eliminated.
Southern sea otters (
Figure 5.1
, left) live in giant kelp forests (Figure 5.1, right) in shallow waters along parts of the Pacific coast of North America. Most of the members of this endangered species are found off the California coast between the cities of Santa Cruz and Santa Barbara.
Figure 5.1
An endangered southern sea otter in Monterey Bay, California (USA) uses a stone to crack the shells of the clams that it feeds on (left). It lives in a bed of seaweed called giant kelp (right).
Left: Kirsten Wahlquist/Dreamstime.com.; Right: Paul Whitted/ Shutterstock.com.
Southern sea otters are fast and agile swimmers that dive to the ocean bottom looking for shellfish and other prey. They swim on their backs on the ocean surface and use their bellies as a table to eat their prey (
Figure 5.1, left). Each day, a sea otter consumes 20–35% of its weight in clams, mussels, crabs, sea urchins, abalone, and other species of bottom-dwelling organisms. Their thick, dense fur traps air bubbles and keeps them warm.
An estimated 16,000 southern sea otters once lived in California’s coastal waters. By the early 1900s, they had been hunted almost to extinction in this region by fur traders who killed them for their luxurious fur. Commercial fishers also killed the sea otters because they competed with them for valuable abalone and other shellfish.
The southern sea otter population grew from a low of 50 in 1938 to 1,850 in 1977 when the U.S. Fish and Wildlife listed the species as endangered. In 2018, there were 3,128 otters.
Why should we care about the southern sea otters of California? One reason is ethical:
Many
people believe it is wrong to allow human activities to cause the extinction of a species. Another reason is that people love to look at these appealing and highly intelligent animals as they play in the water. As a result, the otters
help
to generate millions of dollars a year in tourism revenues. A third reason—and a key reason in our study of environmental science—is that biologists classify the southern sea otter as a keystone species (see
Chapter 4
). Scientists hypothesize that in the absence of southern sea otters, sea urchins and other kelp-eating species would probably destroy the Pacific coast kelp forests and much of the rich biodiversity they support.
Biodiversity is an important part of the earth’s natural capital and is the focus of one of the three scientific principles of sustainability. In this chapter, we look at how species interact and help control one another’s population sizes. We also explore how communities, ecosystems, and populations of species respond to changes in environmental conditions.
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5.1aCompetition for Resources
Ecologists have identified five basic types of interactions among species as they share limited resources such as food, shelter, and space. These types of interactions are called interspecific competition, predation, parasitism, mutualism, and commensalism. They each have a role in limiting the population size and resource use of the interacting species in an ecosystem.
Competition is the most common interaction among species. It occurs when members of one or more species try to use the same limited resources such as food, water, light, and space. Competition between different species is called
interspecific competition
. It plays a larger role in most ecosystems than intraspecific competition—competition among members of the same species.
When two species compete with one another for the same resources, their ecological niches (
Figure 4.9
) overlap. The greater this overlap, the more they compete for key resources. If one species can take over the largest share of one or more key resources, each of the other competing species must move to another area (if possible), suffer a population decline, or become extinct in that area. The niches of two different species can overlap but they cannot simultaneously fully occupy the same niche, a concept called the
competitive exclusion principle
.
Humans compete with many other species for space, food, and other resources. As our ecological footprints grow and spread, we take over or degrade the habitats of many of those species and deprive them of resources they need to survive.
Species evolve to reduce competition for resources by reducing their niche overlap. An example is
resource partitioning
, which occurs when different species competing for similar scarce resources evolve specialized traits that allow them to “share” the same resources. Sharing resources can mean using parts of the resources, or using the resources at different times or in different ways.
Figure 5.2
shows resource partitioning by insect-eating bird species. Adaptations allow the birds to reduce competition by feeding in different portions of certain spruce trees and by feeding on different insect species.
Figure 5.2
Sharing the wealth: Resource partitioning among five species of insect-eating warblers in the spruce forests of the U.S. state of Maine. Each species spends at least half its feeding time in its associated yellow-highlighted areas of these spruce trees.
After R. H. MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,” Ecology 36:533–536, 1958.
Another example of resource partitioning through natural selection involves birds called honeycreepers that live in the U.S. state of Hawaii (
Figure 5.3
).
Figure 4.10
shows how the evolution of specialized feeding niches has reduced competition for resources among bird species in a coastal wetland.
Figure 5.3
Specialist species of honeycreepers: Through natural selection, different species of honeycreepers have shared resources by evolving specialized beaks to take advantage of certain types of food such as insects, seeds, fruits, and nectar from certain flowers.
Question:
1. Look at each bird’s beak and guess what sort of food that bird might eat.
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5.1bPredation
In
predation
, a member of one species that feeds on all or part of a member of another species is called a
predator
, while the species that is fed upon is called the
prey
. Together, they are engaged in a
predator–prey relationship
(
Figure 5.4
). Predation has a strong effect on the population sizes of the competing species.
Figure 5.4
Predator–prey relationship: This brown bear (the predator) in the U.S. state of Alaska has captured and will feed on this salmon (the prey).
Steve Hilebrand/U.S. Fish and Wildlife Service
Connections
Grizzly Bears and Moths
During the summer months, the grizzly bears of the Greater Yellowstone ecosystem in the western United States eat huge amounts of army cutworm moths, which huddle in masses high on remote mountain slopes. In this predator–prey interaction, one grizzly bear can dig out and lap up as many as 40,000 cutworm moths in a day. Consisting of 50–70% fat, the moths offer a nutrient that the bear can store in its fatty tissues and draw on during its winter hibernation.
In a giant kelp forest ecosystem, sea urchins prey on kelp, a type of seaweed (
Science Focus 5.1
). As a keystone species, southern sea otters (
Core Case Study
) prey on the sea urchins and prevent them from destroying the kelp forests. An adult southern sea otter can eat as many as 1,500 sea urchins a day.
Science Focus 5.1
Threats to Kelp Forests
A kelp forest contains large concentrations of seaweed called giant kelp. Anchored to the ocean floor, its long blades grow toward the sunlit surface waters (Figure 5.1, right). Under good conditions, the blades can grow 0.6 meter (2 feet) in a day and the plant can grow as tall as a 10-story building. The kelp blades are flexible and can survive all but the most violent storms and waves.
Kelp forests support many marine plants and animals and are one of the most biologically diverse marine ecosystems. These forests also reduce shore erosion by blunting the force of incoming waves and trapping some of the outgoing sand.
Sea urchins, such as the purple urchin (
Figure 5.A
), prey on kelp plants. Large populations of these predators can rapidly devastate a kelp forest because they eat the bases of young kelp plants. Scientific studies by biologists, including James Estes of the University of California at Santa Cruz, indicate that the southern sea otter (Core Case Study) is a keystone species that helps sustain kelp forests by controlling populations of purple and other sea urchin species.
Figure 5.A
The purple sea urchin inhabits the coastal waters of the U.S. state of California and feeds on kelp.
Kokhanchikov/ Shutterstock.com
Another threat to kelp forests is polluted water running off the land. The pollutants in runoff can include pesticides and herbicides that can kill kelp plants and other species and upset the food webs in these aquatic forests. Another runoff pollutant is fertilizer. Its plant nutrients (mostly nitrates) can cause excessive growth of algae and other aquatic plants. This growth blocks some of the sunlight needed to support the growth of giant kelp.
Some scientists warn that the warming of the world’s oceans is a growing threat to kelp forests, which require cool water. If coastal waters get warmer during this century, as projected by climate models, many or most of California’s coastal kelp forests could disappear.
Critical Thinking
1. List three ways in which we could reduce the degradation of giant kelp forest ecosystems.
Predators use a variety of ways to capture prey. Herbivores can walk, swim, or fly to the plants they feed on. Many carnivores, such as cheetahs, use their speed to chase down and kill prey, such as zebras. Eagles and hawks have keen enough eyesight to spot their prey from the air as they fly. Some predators such as female African lions work in groups to capture large or fast-running prey.
Other predators use camouflage to hide in plain sight and ambush their prey. For example, praying mantises (see
Figure 4.4
, right sit on flowers or plants of a color similar to their own and ambush visiting insects. White ermines (a type of weasel), snowy owls, and arctic foxes (
Figure 5.5
) hunt their prey in snow-covered areas. Some predators use chemical warfare to attack their prey. For example, some spiders and poisonous snakes use venom to paralyze their prey and to defend against their predators.
Figure 5.5
A white arctic fox hunts its prey by blending into its snowy background to avoid being detected.
Paul Nicklen/National Geographic Image Collection
Prey species have evolved many ways to avoid predators. Some can run, swim, or fly fast and some have highly developed senses of sight, sound, or smell that alert them to the presence of predators. Other adaptations include protective shells (abalone and turtles), thick bark (giant sequoia trees), spines (porcupines and sea urchins), and thorns (cacti and rose bushes).
Other prey species use camouflage to blend into their surroundings. Some insect species resemble twigs (
Figure 5.6a
), or bird droppings on leaves. A leaf insect can be almost invisible against its background (
Figure 5.6b
), as can an arctic hare in its white winter fur.
Figure 5.6
These prey species have developed specialized ways to avoid their predators: (a, b) camouflage, (c, d, e) chemical warfare, (d, e, f) warning coloration, (f) mimicry, (g) deceptive looks, and (h) deceptive behavior.
Prey species also use chemical warfare. Some discourage predators by containing or emitting chemicals that are poisonous (oleander plants), irritating (stinging nettles and bombardier beetles,
Figure 5.6c
), foul smelling (skunks and stinkbugs), or bad tasting (buttercups and monarch butterflies,
Figure 5.6d
). When attacked, some species of squid and octopus emit clouds of black ink, allowing them to escape by confusing their predators.
Learning from Nature
Researchers have studied the bombardier beetle’s high-pressure combustion chamber in its abdomen, used to expel a poison that forces predators to vomit the beetle after eating it. Engineers hope to apply this research to industrial or medical spray technology.
Many bad-tasting, bad-smelling, toxic, or stinging prey species flash a warning coloration that eating them is risky. Examples are the brilliantly colored, foul–tasting monarch butterflies (Figure 5.6d) and poisonous frogs (
Figure 5.6e
). When a bird eats a monarch butterfly, it usually vomits and learns to avoid monarchs.
Some butterfly species gain protection by looking and acting like other, more dangerous species, a protective device known as mimicry. For example, the nonpoisonous viceroy butterfly (
Figure 5.6f
) mimics the monarch butterfly. Other prey species use behavioral strategies to avoid predation. Some attempt to scare off predators by puffing up (blowfish), spreading their wings (peacocks), or mimicking a predator (
Figure 5.6h
). Some moths have wings that look like the eyes of much larger animals (
Figure 5.6g
). Other prey species gain some protection by living in large groups such as schools of fish and herds of antelope.
Biologist Edward O. Wilson (
Individuals Matter 4.1
) proposed two criteria for evaluating the dangers posed by various brightly colored animal species. First, if they are small and strikingly beautiful, they are probably poisonous. Second, if they are strikingly beautiful and easy to catch, they are probably deadly.
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5.1cCoevolution
Over time, a prey species develops traits that make it more difficult to catch. Its predators then face selection pressures that favor traits that increase their ability to catch their prey. Then the prey species must get better at eluding the more effective predators.
This back-and-forth adaptation is called
coevolution
, a natural selection process in which changes in the gene pool of one species lead to changes in the gene pool of another species. It can play an important role in controlling population growth of predator and prey species. When populations of two species interact as predator and prey over a long time, coevolution can help the predator succeed and it can help the prey avoid being eaten.
For example, bats prey on certain species of moths (
Figure 5.7
) that they hunt at night using echolocation. They emit pulses of high-frequency sound that bounce off their prey and capture the returning echoes that tell them where their prey is located. Over time, certain moth species have evolved ears that are sensitive to the sound frequencies that bats use to find them. When they hear these frequencies, they drop to the ground or fly evasively. Some bat species evolved ways to counter this defense by changing the frequency of their sound pulses. In turn, some moths evolved their own high-frequency clicks to jam the bats’ echolocation systems. Some bat species then adapted by turning off their echolocation systems and using the moths’ clicks to locate their prey. This is a classic example of coevolution.
Figure 5.7
Coevolution: This bat is using ultrasound to hunt a moth. As the bats evolve traits to increase their chances of getting a meal, the moths evolve traits to help them avoid being eaten.
Michael Duham/Minden Pictures/Superstock
Learning from Nature
Bats and dolphins use echolocation to navigate and locate prey in the darkness of night and in the ocean’s murky water. Scientists are studying how they do this to improve our sonar systems, sonic imaging tools for detecting underground mineral deposits, and medical ultrasound imaging systems.
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5.1dParasitism, Mutualism, and Commensalism
Parasitism
occurs when one species (the parasite) lives in or on another organism (the host). The parasite benefits by extracting nutrients from its host. The parasite weakens its host but rarely kills it, since doing so eliminates the source of its benefits. Parasites can be plants, animals, or microorganisms.
Tapeworms are parasites that live part of their life cycle inside their hosts. Others such as mistletoe plants and blood-sucking sea lampreys (
Figure 5.8
) attach themselves to the outsides of their hosts and suck nutrients from them. Some parasites move from one host to another (fleas and ticks) while others (such as certain protozoa) spend their adult lives within a single host. Parasites help keep their host populations in check.
Figure 5.8
Parasitism: This blood-sucking, parasitic sea lamprey has attached itself to an adult lake trout from one of the Great Lakes of the United States and Canada.
Great Lakes Fishery Commission
In
mutualism
, two species interact in ways that benefit both species by providing each with food, shelter, or some other resource. An example is pollination of flowering plants by species such as honeybees, hummingbirds, and butterflies that feed on the nectar of flowers. These pollinators get food in the form of nectar and spread the pollen from flower to flower, which helps the flower species produce seeds and reproduce.
Figure 5.9
shows an example of a mutualistic relationship that combines nutrition and protection. It involves birds that ride on the backs or heads of large animals such as elephants, rhinoceroses, and impalas. The birds remove and eat parasites and pests (such as ticks and flies) from the animals’ bodies and often make noises warning the animals when predators are approaching.
Figure 5.9
Mutualism: This red-billed oxpecker feeds on parasitic ticks that infest animals such as this impala and warns of approaching predators.
Uwe Bergwitz/ Shutterstock.com
Another example of mutualism involves clownfish, which usually live within sea anemones (see chapter-opening photo), whose tentacles sting and paralyze most fish that touch them. The clownfish, which are not harmed by the tentacles, gain protection from predators and feed on the waste matter left from the anemones’ meals. The sea anemones benefit because the clownfish protect them from some of their predators and parasites.
Mutualism might appear to be a form of cooperation between species. However, each species is acting for its own survival.
Commensalism
is an interaction between two species in which one species benefits and the other species is unaffected. For example, plants called epiphytes (air plants), attach themselves to the trunks or branches of trees (
Figure 5.10
) in tropical and subtropical forests. The epiphytes gain better access to sunlight, water from the humid air and rain, and nutrients falling from the tree’s upper leaves and limbs. Their presence apparently does not harm the tree. Similarly, birds benefit by nesting in trees, generally without harming them.
Figure 5.10
Commensalism: This pitcher plant is attached to a branch of a tree without penetrating or harming the tree. This carnivorous plant feeds on insects that become trapped inside it.
all_about_people/ Shutterstock.com
Succession
Watch this animation to see the difference between primary and secondary succession.
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5.2aEcological Succession
The types and numbers of species in biological communities and ecosystems change in response to changing environmental conditions. The gradual change in species composition in a given community or ecosystem is called
ecological succession
.
There are two major types of ecological succession, depending on the conditions present at the beginning of the process.
Primary ecological succession
involves the gradual establishment of communities of different species in lifeless areas—in terrestrial systems with no soil or in aquatic systems with no bottom sediments. Examples include bare rock exposed by a retreating glacier (
Figure 5.11
), an abandoned highway or parking lot, and a newly created shallow pond or lake (
Figure 5.12
). Primary succession can take hundreds to thousands of years because of the need to build up fertile soil or aquatic sediments to provide the nutrients needed to establish a community of producers.
Figure 5.11
Primary ecological succession: Over almost a thousand years, these plant communities developed, starting on bare rock exposed by a retreating glacier on Isle Royal, Michigan (USA), in western Lake Superior. The details of this process vary from one site to another.
Figure 5.12
Primary ecological succession in a lake basin in which sediments and plants have been gouged out by a glacier. When the glacier melts, the lake basin begins accumulating sediments and plant and animal life. Over hundreds to thousands of years, the lake can fill with sediments and become a terrestrial habitat.
Pioneer species
are the first species to occupy the barren environment and are often carried there by wind or water. Common pioneer species are mosses and lichens (Figure 5.12) because they can grow on rock. They spread and break the rock into pieces that start the long soil formation process. When they die and decompose, they provide nutrients for the thin soil layer.
The other, more common type of ecological succession is called
secondary ecological succession
, in which a community or ecosystem develops on the site of an existing community or ecosystem and replaces or adds to the existing set of resident species. This type of succession begins in an area where an ecosystem has been disturbed, removed, or destroyed, but where some soil or bottom sediment remains. Candidates for secondary succession include abandoned farmland (
Figure
5.13
), burned or cut forests, heavily polluted streams, and flooded land. Because some soil or sediment is present, new vegetation can begin to grow, usually within a few weeks. On land, growth begins with the germination of seeds already in the soil and seeds imported by wind or in the droppings of birds and other animals.
Figure 5.13
Secondary ecological succession: Natural restoration of disturbed land on an abandoned farm field in the U.S. state of North Carolina. It took 150 to 200 years after the farmland was abandoned for the area to become covered with a mature oak and hickory forest. Primary and secondary ecological succession are examples of natural ecological restoration.
Succession
Watch this animation to see the difference between primary and secondary succession.
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Ecological succession is an important ecosystem service that tends to enrich the biodiversity of communities and ecosystems by increasing species diversity and interactions among species. Such interactions enhance an ecosystem’s sustainability by promoting population control and by increasing the complexity of food webs, which enhances energy flow and nutrient cycling.
Ecologists have identified three factors that affect how and at what rate ecological succession occurs. One is facilitation, in which one set of species makes an area suitable for species with different niche requirements, and often less suitable for itself. For example, as lichens and mosses gradually build up soil on a rock in primary succession, herbs and grasses can move in and crowd out the lichens and mosses (
Figure 5.11).
A second factor is inhibition, in which some species hinder the establishment and growth of other species. For example, needles dropping off some pine trees make the soil beneath the trees too acidic for most other plants to grow there. A third factor is tolerance, in which plants in the late stages of succession succeed because they are not in direct competition with other plants for key resources. Shade-tolerant plants, for example, can live in shady forests because they do not need as much sunlight as the trees above them do (
Figures 5.11
and 5.13).
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5.2bIs There a Balance of Nature?
According to the traditional view, ecological succession proceeds in an orderly sequence along an expected path until a certain stable type of climax community (Figures 5.11 and 5.13), which is assumed to be in balance with its environment, occupies an area. This equilibrium model of succession is what ecologists once meant when they talked about the balance of nature.
Over the last several decades, many ecologists have changed their views about balance and equilibrium in nature based on ecological research. There is a general tendency for succession to lead to more complex, diverse, and presumably more resilient ecosystems that can withstand changes in environmental conditions if the changes are not too large or too sudden. However, the current scientific view is that we cannot predict a given course of succession or view it as inevitable progress toward an ideally adapted climax plant community or ecosystem. Rather, ecological succession reflects the ongoing struggle by different species for enough light, water, nutrients, food, space, and other key resources. In other words, research shows that there is no balance of nature consisting of a permanent and stable state.
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5.2cLiving Systems Are Sustained through Constant Change
All living systems, from a cell to the biosphere, are constantly changing in response to changing environmental conditions. Living systems have processes that interact to provide some degree of stability, or sustainability. However, this stability, or the capacity to withstand external stress and disturbance, is maintained by constant change in response to changing environmental conditions. Nature is dynamic, not static, and is not fragile as revealed by how the earth’s life has changed and evolved for 3.8 billion years in response to drastic changes in environmental conditions.
Ecologists distinguish between two aspects of sustainability in ecosystems.
Ecological inertia
, or
persistence
is the ability of an ecosystem to survive moderate disturbances. A second factor,
resilience
, is the ability of an ecosystem to be restored through secondary ecological succession after a severe disturbance.
Evidence suggests that some ecosystems have one of these properties but not the other. However, once a large tract of tropical rain forest is cleared or severely damaged, the resilience of the degraded forest ecosystem may be so low that the degradation reaches an ecological tipping point. Once it exceeds that point, the forest might not be restored by secondary ecological succession. One reason is that most of the nutrients in a tropical rain forest are stored in its vegetation, not in the topsoil. Once the nutrient-rich vegetation is gone, frequent rains can remove most of the remaining soil nutrients and thus prevent the return of a tropical rain forest to a large cleared area.
By contrast, grasslands are much less diverse than most forests. Thus, they have low inertia and can burn easily. Because most of their plant matter is stored in underground roots, these ecosystems have high resilience and can recover quickly after a fire because their root systems produce new grasses. Grassland can be destroyed only if its roots are plowed up and something else is planted in its place, or if it is severely overgrazed by livestock or other herbivores.
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5.3aPopulations Can Grow, Shrink, or Remain Stable
A population is a group of interbreeding individuals of the same species (
Figure 5.14
). Most populations live together in clumps or groups such as packs of wolves, schools of fish (Figure 5.14), and flocks of birds. Living in groups allows them to cluster where resources are available, provides some protection from predators, and helps some predator species to find and capture prey.
Figure 5.14
A population, or school, of Big Eye Trevally Jack in Baja California Sur, Mexico.
Leonardo Gonzalez/ Shutterstock.com
Population size
is the number of individual organisms in a population at a given time. Four variables—births, deaths, immigration, and emigration—govern changes in population size. A population increases through birth and immigration (the arrival of individuals from outside the population). Populations decrease through death and emigration (the departure of individuals from the population):
Scientists use sampling techniques to estimate the sizes of large populations of species such as oak trees that are spread over a large area and squirrels that move around and are hard to count. Typically, they count the number of individuals in one or more small sample areas and use this information to estimate the number of individuals in a larger area.
A population’s
age structure
—its distribution of individuals among various age groups—can have a strong effect on how rapidly its numbers grow or decline. Age groups are usually described in terms of organisms not mature enough to reproduce (the pre-reproductive stage), those capable of reproduction (the reproductive stage), and those too old to reproduce (the post-reproductive stage).
The size of a population will likely increase if it is made up mostly of individuals in their reproductive stage, or soon to enter this stage. In contrast, the size of a population dominated by individuals in their post-reproductive stage will tend to decrease over time.
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5.3bSeveral Factors Can Limit Population Size
Each population in an ecosystem has a
range of tolerance
—a range of variations in its physical and chemical environment within which it is most likely to survive. For example, a trout population (
Figure 5.15
) will thrive within a narrow band of temperatures (optimum level or range), although a few individuals can survive above and below that band (Figure 5.15). If the water becomes too hot or too cold, none of the trout can survive.
Figure 5.15
Range of tolerance for a population of trout to changes in water temperature.
Various physical or chemical factors can determine the number of organisms in a population and how fast a population grows or declines. Sometimes one or more factors, known as
limiting factors
, are more important than other factors in regulating population growth.
Learning from Nature
Biomimicry researchers are hoping to learn how plants that have a high tolerance for salty seawater can teach us how to design better ways of providing fresh drinking water in drought-prone areas.
On land, precipitation often is the limiting factor.
Low
precipitation levels in desert ecosystems limit desert plant growth. Lack of key soil nutrients limits the growth of plants, which in turn limits populations of animals that eat plants, and animals that feed on such plant-eating animals.
Limiting physical factors for populations in aquatic systems include water temperature (Figure 5.15), depth, and clarity (allowing for more or less sunlight). Other important factors are nutrient availability, acidity, salinity, and the level of oxygen gas in the water (dissolved oxygen content).
Too much of a physical or chemical factor can also be limiting. For example, too much water or fertilizer can kill land plants. If acidity levels are too high in an aquatic environment, some of its organisms can be harmed.
An additional factor that can limit the sizes of some populations is
population density
, the number of individuals in a population found within a defined area or volume. Density-dependent factors are variables that become more important as a population’s density increases. In a dense population, parasites and diseases can spread more easily, resulting in higher death rates, and competition for resources such as food and water can intensify. On the other hand, a higher population density can help sexually reproducing individuals to find mates more easily to produce offspring. Other factors such as drought, and climate change are considered density-independent factors, because they can affect population sizes regardless of density.
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5.3cNo Population Can Grow Indefinitely: J-Curves and S-Curves
The populations of some species, such as bacteria and many insect species, have an ability to increase their numbers exponentially. For example, with no controls on its population growth, a species of bacteria that can reproduce every 20 minutes would generate enough offspring to form a 0.3-meter-deep (1-foot-deep) layer over the surface of the entire earth in only 36 hours. Plotting such numbers against time yields a J-shaped curve of exponential growth (
Figure 5.16
, left). Members of such populations typically reproduce at an early age, have many offspring each time they reproduce, and reproduce many times with short intervals between generations.
Figure 5.16
According to this idealized mathematical model, populations of species can undergo exponential growth represented by a J-shaped curve (left) when resource supplies are plentiful. As resource supplies become limited, a population undergoes logistic growth, represented by an S-shaped curve (right), when the size of the population approaches the carrying capacity of its habitat.
However, there are always limits to population growth in nature. Research reveals that a rapidly growing population of any species eventually reaches some size limit imposed by limiting factors. These factors include sunlight, water, temperature, space, nutrients, or exposure to predators or infectious diseases. Environmental resistance is the sum of all such factors in a habitat.
Limiting factors largely determine an area’s carrying capacity, the maximum population of a given species that a particular habitat can sustain indefinitely. As a population approaches the carrying capacity of its habitat, the J-shaped curve of its exponential growth (Figure 5.16, left) is converted to an S-shaped curve of logistic growth, or growth that often fluctuates around the carrying capacity of its habitat (Figure 5.16, right).
However, the rate of population growth and the carrying capacity for a population are not fixed and can rise or fall as environmental conditions change the factors that promote and limit the population’s growth. Nature is constantly changing and is never in balance. In other words, the curve in Figure 5.16 is a simplified and idealized mathematical model of the growth rate and carrying capacity of populations in nature.
Some populations do not make a smooth transition from exponential growth to logistic growth. Instead, they use up their resource supplies and temporarily overshoot, or exceed, the carrying capacity of their environment. In such cases, the population suffers a sharp decline, called a dieback, or population crash, unless part of the population can switch to new resources or move to an area that has more resources. Such a crash occurred when reindeer were introduced onto a small island in the Bering Sea in the early 1900s (
Figure 5.17
).
Figure 5.17
Exponential growth, overshoot, and population crash of a population of reindeer introduced onto the small Bering Sea island of St. Paul in 1910.
Data Analysis:
1. By what percentage did the population of reindeer grow between 1923 and 1940?
Patterns of Population growth
Watch this animation to see what factors affect population size.
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5.3dReproductive Patterns
Species vary in their reproductive patterns. Species with a capacity for a high rate of population growth (r) (Figure 5.16, left) are called
r-selected species
. These species tend to have short life spans and produce many, usually small offspring and give them little or no parental care. As a result, many of the offspring die at an early age. To overcome such losses, r-selected species produce large numbers of offspring so a few will likely survive and have many offspring to sustain the species. Examples of r-selected species include algae, bacteria, frogs, and most insects.
Such species tend to be opportunists. They reproduce and disperse rapidly when conditions are favorable or when a disturbance such as a fire or clear-cutting of a forest opens up a new habitat or niches for invasion. Once established, their populations may crash because of unfavorable changes in environmental conditions or invasion by more competitive species. This explains why most opportunist species go through irregular and unstable boom-and-bust cycles in their population sizes.
At the other extreme are
K-selected species
. They tend to reproduce later in life, have few offspring, and have long life spans. Typically, the offspring of K-selected mammal species develop inside their mothers (where they are safe). After birth, they mature slowly and one or both parents care for and protect them. In some cases, they live in herds or groups until they reach reproductive age.
The population size of K-selected species tends to be near the carrying capacity (K) of its environment (Figure 5.16, right). Examples of K-selected species include most large mammals such as elephants, whales, and humans, birds of prey, large and long-lived plants such as the saguaro cactus, and most tropical rain forest trees. Many of these species—especially those with low reproductive rates, such as elephants, sharks, giant redwood trees, and California’s southern sea otters (Core Case Study and
Science Focus 5.2
)—are vulnerable to extinction. Most organisms have reproductive patterns between the extremes of r-selected and K-selected species.
Table 5.1
compares typical traits of r-selected and K-selected species.
Table 5.1
Typical Traits of r-Selected and K-Selected Species
|
Trait |
r-Selected Species |
K-Selected Species |
||
|
Reproductive potential |
High |
Low | ||
|
Population growth rate |
Fast |
Slow |
||
|
Time to reproductive maturity |
Short |
Long |
||
|
Number of reproductive cycles |
Many |
Few |
||
|
Number of offspring |
||||
|
Size of offspring |
Small |
Larger |
||
|
Degree of parental care |
||||
|
Life span |
||||
| Population size |
Variable with crashes |
Stable, near carrying capacity |
||
|
Role in environment |
Usually prey |
Usually predators |
Science Focus 5.2
The Future of California’s Southern Sea Otters
The population of southern sea otters (Core Case Study) has fluctuated in response to changes in environmental conditions (
Figure 5.B
). One change was a rise in populations of the orcas (killer whales) that feed on them. Scientists hypothesize that orcas started feeding more on southern sea otters when populations of their normal prey, sea lions and seals, began declining. In addition, between 2010 and 2015, the number of sea otters killed or injured by sharks increased, possibly because warmer ocean water brought some sharks closer to the shore.
Figure 5.B
Changes in the population size of southern sea otters off the coast of the U.S. state of California, 1983–2018.
(Compiled by the authors using data from U.S. Geological Survey.)
Another factor affecting sea otters may be parasites that breed in the intestines of cats. Scientists hypothesize that some southern sea otters are dying because coastal area cat owners flush feces-laden cat litter down their toilets or dump it in storm drains that empty into coastal waters. The feces contain parasites that can infect otters.
Toxic algae blooms also threaten otters. The algae thrive on urea, a nitrogen-containing ingredient in fertilizer that washes into coastal waters. Other pollutants released by human activities include PCBs and other fat-soluble toxic chemicals. These chemicals can kill otters by accumulating to high levels in the tissues of the shellfish that otters eat. Because southern sea otters feed at high trophic levels and live close to the shore, they are vulnerable to these and other pollutants in coastal waters.
Other threats to otters include oil spills from ships. The entire California southern sea otter population could be wiped out by a large oil spill from a single tanker off the central west coast or by the rupture of an offshore oil well, should drilling for oil be allowed off this coast. Some sea otters die when they are trapped in underwater nets and traps for shellfish. Others are killed by boat strikes and gunshots.
Figure 5.B shows the change in the population size of the southern sea otter since 1967, ten years before it was protected as an endangered species. In 2016, the sea otter population was 3,272 the highest it has been since 1985. In 2017, the population was 3,186 and in 2018, it was 3,128. Thus, for three years the sea otter population has averaged above 3,090, which means it can be considered for removal from the federal endangered species list. Such a delisting would be a success story for the U.S. Endangered Species Act and the otters would still be protected under a California state law.
Critical Thinking
1. How would you design a controlled experiment to test the hypothesis that cat litter flushed down toilets might be killing southern sea otters?
The reproductive pattern of a species may give it a temporary advantage. However, the key factor in determining the ultimate population size of a species is the availability of suitable habitat with adequate resources. Changes in habitat or other environmental conditions can reduce the populations of some species while increasing the populations of other species, such as white-tailed deer in the United States (see Case Study that follows).
Case Study
Exploding White-Tailed Deer Populations in the United States
By 1900, habitat destruction and uncontrolled hunting had reduced the white-tailed deer (
Figure 5.18
) population in the United States to about 500,000 animals. In the 1920s and 1930s, laws were passed to protect the remaining deer. Hunting was restricted and predators, including wolves and mountain lions that preyed on the deer, were nearly eliminated.
Figure 5.18
White-tailed deer populations in the United States have been growing.
Roy Toft/National Geographic Image Collection
These protections worked, perhaps too well for some suburbanites and farmers. Today there are over 30 million white-tailed deer in the United States. During the last 50 years, suburbs have expanded and many Americans have moved into the wooded habitat of deer. The gardens and landscaping around their homes provide deer with flowers, shrubs, garden crops, and other plants they like to eat.
Deer prefer to live in the edge areas of forests and woodlots for security and go to nearby fields, orchards, lawns, and gardens for food, so white-tailed deer populations have soared in suburban areas.
In woodlands, larger populations of the deer are consuming native ground-cover vegetation, which has allowed nonnative weed species to take over and upset ecosystem food webs. The deer also help to spread Lyme disease (carried by deer ticks) to humans. In addition, each year about 1.5 million deer–vehicle collisions injure thousands and kill more than 160 people per year, on average—the highest human death toll from encounters with any wild animal in the United States.
There are no easy solutions to the deer population problem in the suburbs. Changes in hunting regulations that allow for the killing of more female deer have cut down the overall deer population. However, this has had a limited effect on deer populations in suburban areas because it is too dangerous to allow widespread hunting with guns in such populated communities. Some areas have hired experienced and licensed archers who use bows and arrows to help reduce deer numbers, being careful not to endanger nearby residents.
Some communities spray the scent of deer predators or of rotting deer meat in edge areas to scare off deer. Others scare off deer by using electronic equipment that emits high-frequency sounds that humans cannot hear. Some homeowners surround their gardens and yards with high, black plastic mesh fencing.
Deer can be trapped and moved from one area to another, but this is expensive and must be repeated whenever they move back into an area. In addition, there are questions concerning where to move the deer and how to pay for such programs.
Darts loaded with contraceptives can be shot into female deer to hold down their birth rates, but this is expensive and must be repeated every year. Another approach is to trap dominant males and use chemical injections to sterilize them. However, this is costly and will require years of testing. In addition, ethical questions about this approach would have to be considered.
Meanwhile, suburbanites can expect deer to chow down on their shrubs, flowers, and garden plants unless they can protect their properties with fences, repellents, or other methods. Suburban dwellers could also stop planting trees, shrubs, and flowers that attract deer around their homes.
Critical Thinking
1. If the earth experiences significant warming during this century as projected, is this likely to favor r-selected or K-selected species? Explain.
Critical Thinking
1. Some people blame the white-tailed deer for invading farms and suburban yards and gardens to eat food that humans have made easily available to them. Others say humans are mostly to blame because they have invaded deer territory, eliminated most of the predators that kept deer populations under control, and provided the deer with plenty to eat in their lawns, gardens, and crop fields. Which view do you hold? Why? Do you see a solution to this problem?
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5.3eSurvivorship Curves
Individuals of species with different reproductive strategies tend to have different life expectancies. This can be illustrated by a
survivorship curve
, which shows the percentages of the members of a population surviving at different ages. There are three generalized types of survivorship curves: late loss, early loss, and constant loss (
Figure 5.19
). A late loss population (K-selected species such as elephants and rhinoceroses) typically has high survivorship to a certain age, and then high mortality. A constant loss population (such as many songbirds) typically has a constant death rate at all ages. For an early loss population (many r-selected species and annual plants), survivorship is low early in life. These generalized survivorship curves only approximate the realities of nature.
Figure 5.19
Survivorship curves for populations of different species, obtained by showing the percentages of the members of a population surviving at different ages.
Critical Thinking:
1. Which type of survivorship curve applies to the human species?
Top: Gualtiero boffi/ Shutterstock.com. Center: IrinaK/ Shutterstock.com. Bottom: ultimathule/ Shutterstock.com.
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5.3fHumans Are Not Exempt from Nature’s Population Controls
Humans are not exempt from population crashes. In 1845, Ireland experienced such a crash after a fungus destroyed its potato crop. About 1 million people died from hunger or diseases related to malnutrition and millions more migrated to other countries, sharply reducing the Irish population.
During the 14th century, bubonic plague spread through densely populated European cities and killed at least 25 million people—one-third of the European population. The bacterium that causes this disease normally lives in rodents. It was transferred to humans by fleas that fed on infected rodents and then bit humans. The disease spread like wildfire through crowded cities, where sanitary conditions were poor and rats were abundant. Today several antibiotics can be used to treat bubonic plague.
So far, technological, social, and other cultural changes have expanded the earth’s carrying capacity for the human species. We have used large amounts of energy and matter resources to occupy formerly uninhabitable areas. We have expanded agriculture and controlled the populations of other species that compete with us for resources. Some say we can keep expanding our ecological footprint in this way indefinitely because of our technological ingenuity. Others say that at some point, we will reach the limits that nature eventually imposes on any population that exceeds or degrades its resource base. We discuss these issues in
Chapter 6
.
Big Ideas
· Certain interactions among species affect their use of resources and their population sizes.
· The species composition and population sizes of a community or ecosystem can change in response to changing environmental conditions through a process called ecological succession.
· No population can escape natural limiting factors and grow indefinitely.
Patterns of Population growth
Watch this animation to see what factors affect population size.
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Tying It All TogetherSouthern Sea Otters and Sustainability
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The southern sea otters of California are part of a complex ecosystem made up of large underwater kelp forests, bottom-dwelling creatures, and other species that depend on one another for survival. The sea otters act as a keystone species, mostly by feeding on sea urchins and keeping them from destroying the kelp.
In this chapter, we focused on how biodiversity promotes sustainability, provides a variety of species to restore damaged ecosystems through ecological succession, and limits the sizes of populations. Populations of most plants and animals depend, directly or indirectly, on solar energy, and all populations play roles in the cycling of nutrients in the ecosystems where they live. In addition, the biodiversity in different terrestrial and aquatic ecosystems provides alternative paths for energy flow and nutrient cycling, better opportunities for natural selection as environmental conditions change, and natural population control mechanisms. When we disrupt these paths, we violate the three scientific principles of sustainability.
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Chapter Review
Chapter Review
Critical Thinking
1. What difference would it make if the southern sea otter (
Core Case Study) became extinct primarily because of human activities? What are three things we could do to help prevent the extinction of this species?
2. Use the second law of thermodynamics (
Chapter 2
) and the concept of food chains and food webs to explain why predators are generally less abundant than their prey.
3. How would you reply to someone who argues that we should not worry about the effects that human activities have on natural systems because ecological succession will repair whatever damage we do?
4. How would you reply to someone who contends that efforts to preserve species and ecosystems are not worthwhile because nature is largely unpredictable?
5. What is the reproductive strategy of most species of insect pests and harmful bacteria? Why does this make it difficult for us to control their populations?
6. List three examples of how your life might be affected if changing environmental conditions favor r-selected species during the latter half of this century.
7. List two factors that may limit human population growth in the future. Do you think that we are close to reaching those limits? Explain.
8. If the human species were to suffer a population crash, name three species that might move in to occupy part of our ecological niche. What are three species that would likely decline as a result? Explain why these other species would decline.
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Chapter Review
Doing Environmental Science
1. Visit a nearby land area, such as a partially cleared or burned forest, grassland, or an abandoned crop field, and record signs of secondary ecological succession. Take notes on your observations and formulate a hypothesis about what sort of disturbance led to this succession. Include your thoughts about whether this disturbance was natural or caused by humans. Study the area carefully to see whether you can find patches that are at different stages of succession and record your thoughts about what sorts of disturbances have caused these differences. You might want to research the topic of ecological succession in such an area.
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Chapter Review
Data Analysis
The graph below shows changes in the size of an Emperor penguin population in terms of numbers of breeding pairs on the island of Terre Adelie in the Antarctic. Scientists used this data along with data on the penguins’ shrinking ice habitat to project a general decline in the island’s Emperor penguin population, to the point where they will be endangered in 2100. Use the graph to answer the following questions.
1. If the penguin population fluctuates around the carrying capacity, what was the approximate carrying capacity of the island for the penguin population from 1960 to 1975? What was the approximate carrying capacity of the island for the penguin population from 1980 to 2010?
2. What was the overall percentage decline in the penguin population from 1975 to 2010?
3. What is the projected overall percentage decline in the penguin population between 2010 and 2100?
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Chapter Introduction
A clownfish gains protection by living among sea anemones and helps
protect the anemones from some of their
predators.
cbpix/
Shutterstock.com
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Core Case Study
The Southern Sea
Otter: A Species in Recovery
Learning Objective
·
LO 5.1
Explain what could happen to the Pacific coast kelp
forest
ecosystem if the southern sea otters were eliminated.
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Chapter Introduction
A clownfish gains protection by living among sea anemones and helps
protect the anemones from some of their predators.
cbpix/ Shutterstock.com
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Core Case StudyThe Southern Sea
Otter: A Species in Recovery
Learning Objective
LO 5.1Explain what could happen to the Pacific coast kelp forest
ecosystem if the southern sea otters were eliminated.
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content
Chapter Introduction
Slum area (bottom) in Mumbai, India
ZUMA Press, Inc./Alamy Stock Photo
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Core Case Study
Planet Earth:
Population 7.6 Billion
Learning Objectives
·
LO 6.1
Describe the
growth of the human population in terms of numbers
of years between doublings.
· LO 6.2List three major factors that account for the rapid rise of the human population.
It took about 200,000 years for the human population to reach an estimated 2 billion. It took less than 50 years to add the second 2 billion people (by about
19
74), and 25 years to add the third 2 billion (by 1999). Nineteen years later, in 201
8
, the earth had 7.6 billion people. In 20
18
, the three most populous countries, in order, were China with
1.
3
9 billion people (
Figure 6.1
), India with 1.37 billion people, and the United States with 328 million people. The United Nations projects that the world’s population will increase to 9.9 billion by 2050—an increase of 2.3 billion people.
Figure 6.1
This crowded street is located in Shanghai, China, the world’s most populous country.
TonyV3112/ Shutterstock.com
Does it matter that there are now 7.6 billion people on the earth—almost 3 times as many as there were in 1950? Does it matter that each day, 249,000 more people show up for dinner and many of them will go hungry? Does it matter that there might be 2.3 billion more of us by 2050? Some say it does not matter, and they contend that we can develop new technologies that could easily support billions more people.
Many scientists disagree and contend that the current exponential growth of the human population (see
Figure 1.12
) is unsustainable because as our population and economies grow, we use more of the earth’s natural resources and our ecological footprints grow. As a result, we degrade the natural capital that keeps us alive and supports our lifestyles and economies.
According to demographers, or population experts, three major factors account for the rapid rise of the human population. First, the emergence of early and modern agriculture about
10
,000 years ago increased food production. Second, additional technologies helped humans expand into almost all of the planet’s climate zones and habitats (see
Figure 1.9
). Third, death rates dropped sharply with improved sanitation and health care and the development of antibiotics and vaccines to control infectious diseases.
What is a sustainable level for the human population? Population experts have made low, medium, and high projections of the human population size by the end of this century (see Figure 1.12). No one knows whether, or for how long, any of these population sizes are sustainable.
In this chapter, we examine trends, environmental impacts, and ways to deal with human population growth and decline.
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6.1aHuman Population Growth
For most of history, the human population grew slowly (see Figure 1.12, left part of curve). However, it has grown rapidly for the last 200 years, resulting in the characteristic J-curve of exponential growth (Figure 1.12, right part of curve).
Demographers, or population experts, recognize three important trends related to the current size, growth rate, and distribution of the human population. First, the rate of population growth decreased in most years since 1965, but the world’s population grew at a rate of 1.20% in 2018 (
Figure 6.2
). This may not seem like much. However, in 2018 this growth rate added about 91 million people to the population—an average of 249,000 more people every day.
Figure 6.2
Global human population size compared with population growth rate, 1950–2018, with projection to 2050 (in blue).
Critical Thinking
:
1. While the annual growth rate of world population has generally dropped since the 1960s, how do you explain the continued growth of the overall population?
(Compiled by the authors using data from United Nations Population Division, U.S. Census Bureau, and Population Reference Bureau.)
2.3 Billion
Projected increase in the world’s population between 2018 and 2050
Second, human population growth is unevenly distributed and this pattern is expected to continue (
Figure 6.3
). About 96% of the 91 million new arrivals on the planet in 2018 were added to the world’s less-developed countries. The other 4% were added to the more developed countries.
Figure 6.3
Most of the world’s population growth between 1950 and 2018 took place in the world’s less-developed countries. This gap is projected to increase between 2018 and 2050.
(Compiled by the authors using data from United Nations Population Division and Population Reference Bureau.)
At least 95% of the 2.3 billion people projected to be added to the world’s population between 2018 and 2050 will be born into the less-developed countries (
Figure 6.3). Most of these countries are not equipped to deal with the pressures of rapid population growth.
Third, people have moved in large numbers from rural areas to urban areas. In 2018, 55% of the world’s people lived in urban areas, and this percentage is increasing. Most of these urban dwellers live in less-developed countries where resources for dealing with rapidly growing populations are limited
Scientists and other analysts have long pondered the question: How long can the human population continue to grow while sidestepping many of the factors that sooner or later limit the growth of any population? These experts disagree over how many people the earth can support indefinitely. So far, advances in food production and health care have prevented sharp population declines, but there is extensive and growing evidence that human activities are depleting and degrading much of the earth’s irreplaceable natural capital.
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6.1bHow Long Can the Human Population Keep Growing?
Are there physical limits to human population growth and economic growth on a finite planet? Some say yes. Others say no.
This debate has been going on since 1
79
8 when Thomas Malthus, a British economist, hypothesized that the human population tends to grow exponentially, while food supplies tend to increase more slowly at a linear rate. However, food production has grown at an exponential rate instead of at a linear rate because of technological advances in industrialized food production.
One view is that we have already exceeded some of those limits, with too many people collectively degrading the earth’s life-support system. To some analysts, the key problem is the large and rapidly growing number of people in less-developed countries, which have 84% of the world’s population. To others, the key factor is overconsumption in affluent, more-developed countries with high rates of resource use per person.
Another view of population growth is that technology has allowed us to overcome the environmental limits that all populations of other species face. According to this view, technological advances have increased the earth’s carrying capacity for the human species. Some analysts point out that average life expectancy in most of the world has been steadily rising despite warnings from some environmental scientists that we are seriously degrading our life-support system.
These analysts argue that because of our technological ingenuity, there are few, if any, limits to human population growth and resource use per person. They believe that we can continue increasing economic growth and avoid serious damage to our life-support systems by making technological advances in areas such as food production and medicine, and by finding substitutes for resources that we are depleting. They see no need to slow the world’s population growth or resource consumption.
Proponents of slowing or stopping population growth point out that currently, we are failing to provide the basic necessities for about 815 million people who struggle to survive on the equivalent of about $1.90 per day and the 2.1 billion people struggling to live on $3.10 or less. This raises a serious question: How will we meet the basic needs of the additional 2.3 billion people projected to be added mostly to less-developed countries between 2018 and 2050?
Proponents of slowing population growth also warn of two potentially serious consequences if we do not sharply lower birth rates. First, death rates could increase because of declining health and environmental conditions and increasing social disruption in some areas, as is happening today in parts of Africa. A worst-case scenario for such a trend is a crash of the human population from more than 7 billion to a more sustainable level of 4 billion or perhaps as low as 2 billion. Second, resource use and degradation of normally renewable resources may intensify as more consumers increase their already large ecological footprints in more-developed countries and in rapidly developing countries such as China, India, and Brazil.
As the human population grows, so does the global human ecological footprint (see Figure 1.9), and the bigger this footprint, the higher the overall impact of humanity on the earth’s natural capital. The 2005 Millennium Ecosystem Assessment concluded that human activities have degraded about 60% of the earth’s ecosystem services. Despite advances in food production and health care that have prevented widespread population declines, we are depleting and degrading much of the earth’s natural capital (see
Figure 1.3
and
Figure 6.4
). We can get away with this for a while, because the earth’s life-support system is resilient. However, such disturbances could reach various tipping points beyond which there could be damaging and long-lasting change (
Science Focus 3.2
).
Figure 6.4
Human activities have altered the natural systems that sustain our lives and economies in at least eight major ways to meet the increasing needs and wants of the growing human population.
Critical Thinking:
Critical Thinking:
Critical Thinking:
Critical Thinking:
1. In your daily living, do you think you contribute directly or indirectly to any of these harmful environmental impacts? Which ones? Explain.
Top: Dirk Ercken/ Shutterstock.com. Center: Fulcanelli/ Shutterstock.com. Bottom: Werner Stoffberg/ Shutterstock.com.
No one knows how close we are to environmental limits that eventually might control the size of the human population primarily by raising the human death rate, according to many scientists. These analysts call for us to confront this vital scientific, political, economic, and ethical issue.
Some say that asking how many people the earth can support indefinitely is asking the wrong question. Instead, they call for us to estimate the planet’s
cultural carrying capacity
—the maximum number of people who could live in reasonable freedom and comfort indefinitely, without decreasing the ability of the earth to sustain future generations.
Critical Thinking
1. Do you think there are environmental limits to human population growth? Explain. If so, how close do you think we are to such limits? Explain.
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6.2aThe Human Population Can Grow, Decline, or Stabilize
The basics of global human population change are simple. When there are more births than deaths, the human population increases; when there are more deaths than births, it decreases. When the number of births equals the number of deaths, population size does not change.
Instead of using the total numbers of births and deaths per year, demographers use the
crude birth rate
(the number of live births per 1,000 people in a population in a given year) and the
crude death rate
(the number of deaths per 1,000 people in a population in a given year).
The human population in a particular area grows or declines through the interplay of three factors: births (fertility), deaths (mortality), and migration. We can calculate the
population change
of an area by subtracting the number of people leaving a population (through death and emigration) from the number entering it (through birth and immigration) during a year:
When births plus immigration exceed deaths plus emigration, a population grows; when the reverse is true, a population declines.
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6.2bFertility Rates
Demographers distinguish between two types of fertility rates. One is the
replacement-level fertility rate
: the average number of children that couples in a population must bear to replace themselves. It is slightly higher than two children per couple (typically 2.1) because some children die before reaching their reproductive years, especially in the world’s poorest countries.
If we were to reach a global replacement-level fertility rate of 2.1 tomorrow, would it bring an immediate halt to human population growth? No, because there is a large number of potential mothers under age 15 who will be moving into their reproductive years.
The second type of fertility rate is the
total fertility rate (TFR)
. It is the average number of children born to the women of childbearing age in a population. It is a key factor affecting human population growth and size.
Between 1955 and 2018, the global TFR dropped from 5.0 to 2.4. Those who support slowing the world’s population growth view this as good news. However, to eventually halt human population growth, the global TFR must drop to and remain at the fertility replacement level of 2.1—the rate necessary for replacing both parents after considering infant mortality.
With a TFR of 4.6, Africa’s population is growing more than twice as fast as any other continent and is projected to more than double from 1.3 billion in 2018 to 2.6 billion in 2050 and continue to grow. By the end of this century, Africa is projected to have 40% of the world’s people. Africa is also the world’s poorest continent.
Estimates of any population’s future numbers can vary considerably, depending mostly on TFR projections. Demographers also have to make assumptions about death rates, migration, and a number of other variables. If their assumptions are wrong, their population forecasts can be inaccurate (
Science Focus 6.1
).
Science Focus 6.1
Projecting Population Change
Estimates of the human population size in 2050 range from 7.8 billion to 10.8 billion people—a difference of 3 billion. The range of estimates varies because many factors affect birth rates and TFRs.
First, demographers have to determine the reliability of current population estimates. While many more-developed countries such as the United States have reliable estimates of their population size, most countries do not. Some countries deliberately inflate or deflate the numbers for economic or political purposes.
Second, demographers make assumptions about trends in fertility. They might assume that fertility is declining by a certain percentage per year. If this estimate is off by a few percentage points, the resulting percentage increase in population can be magnified over a number of years and be quite different from the projected population size increase.
For example, United Nation (UN) demographers assumed that Kenya’s fertility rate would decline. Based on that, in 2002 they projected that Kenya’s total population would be
44
million by 2050. In reality, the fertility rate rose sharply. As a result, in 2018 the UN revised its projection for Kenya’s population in 2050 to 96 million, which was
14
0% higher than its earlier projection.
Third, population projections are made by a variety of organizations. UN projections are often cited, but the U.S. Census Bureau, the International Institute for Applied Systems Analysis (IIASA), and the U.S. Population Reference Bureau also make projections. Their projections vary because they use differing sets of data and differing methods (
Figure 6.A
).
Figure 6.A
World population projections to 2050 from three different organizations: the UN, the U.S. Census Bureau, and IIASA. Note that the uppermost, middle, and lowermost curves of these five projections are all from the UN, each assuming a different level of fertility.
Data Analysis:
1. What are the ranges (differences between the lowest and highest) in these projections for 2030, 2040, and 2050?
(Compiled by the authors using data from United Nations, U.S. Census Bureau, and IIASA.)
Critical Thinking
1. If you were in charge of the world and making decisions about resource use based on population projections, which of the projections in Figure 6.A would you rely on? Explain.
Case Study
The U.S. Population—Third Largest and Growing
Between 1900 and 2018, the U.S. population grew from 76 million to 328 million. This happened despite oscillations in the country’s TFR (
Figure 6.5
) and population growth rate.
Figure 6.5
Total fertility rates for the United States between 1917 and 2018.
Critical Thinking:
1. The U.S. fertility rate has declined and remained at or below replacement levels since 1972. So why is the population of the United States still increasing?
(Compiled by the authors using data from Population Reference Bureau and U.S. Census Bureau.)
During the period of high birth rates between 1946 and 1964, known as the baby boom, 79 million people were added to the U.S. population. At the peak of the baby boom in 1957, the average TFR was 3.7 children per woman. In most years since 1972, it has been at or below 2.1 children per woman. In 2018, it was 1.8 compared to a global TFR of 2.4. A key factor in this decline is an increase in the average age of a woman at the time when her first child was born.
The drop in the TFR has slowed the rate of population growth in the United States, but the country’s population is still growing. In 2018, about 1.8 million people were added to the U.S. population—1 million due to the fact that there were more births than deaths and 800,000 due to legal immigration.
Since 1
82
0, the United States has admitted almost twice as many legal immigrants and refugees as all other countries combined. The number of legal immigrants (including refugees) has varied during different periods because of changes in immigration laws and rates of economic growth (
Figure 6.6
).
Figure 6.6
Legal immigration to the United States, 1820–2013 (the last year for which data are available). The large increase in immigration since 1989 resulted mostly from the Immigration Reform and Control Act of 1986, which granted legal status to certain illegal immigrants who could show they had been living in the country prior to January 1, 1982.
(Compiled by the authors using data from U.S. Immigration and Naturalization Service, the Immigration Policy Institute, and the Pew Hispanic Center.)
Since 1965, nearly 59 million people have legally immigrated to the United States, most of them from Latin America and Asia, with the government giving preferences for those with technical training or with family members of U.S. citizens. A 2015 study by the U.S. Census Bureau noted that in 2013, China surpassed Mexico as the largest source of new U.S. immigrants.
According to population experts, the country’s influx of immigrants has made the country more culturally diverse and has increased economic growth as these citizens worked and started businesses. The United States has an estimated 10.7 million illegal immigrants. There is controversy over whether to deport those who can be found or to allow these individuals to meet strict criteria for becoming U.S. citizens. Since 2005, the flow of illegal immigrants into the country has been dropping, according to the Pew Research Center.
In addition to the fourfold increase in population since 1900, some amazing changes in lifestyles took place in the United States during the 20th century (
Figure 6.7
), which led to Americans living longer. Along with this came dramatic increases in per capita resource use and much larger total and per capita ecological footprints.
Figure 6.7
Some major changes that took place in the United States between 1900 and 2000.
(Compiled by the authors using data from U.S. Census Bureau and Department of Commerce.)
62 Million
Projected increase in the U.S. population between 2018 and 2050
The U.S. Census Bureau projects that between 2018 and 2050, the U.S. population will likely grow from 328 million to 390 million—an increase of 62 million people. Because of a high per-person rate of resource use and the resulting waste and pollution, each addition to the U.S. population has an enormous environmental impact.
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6.2cFactors That Affect Birth and Fertility Rates
Many factors affect a country’s average birth rate and total fertility rate (TFR). One is the importance of children as a part of the labor force, especially in less-developed countries. Many poor couples in those countries struggle to survive on less than $3.10 a day and some on less than $1.90 a day. Some of these couples have a large number of children to
help
them haul drinking water, gather wood for heating and cooking, and grow or find food. Worldwide, 1 of every 10 children between ages 5 and 17 work to help the family survive (
Figure 6.8
).
Figure 6.8
This young boy spends much of his day carrying bricks.
Zatletic/ Dreamstime.com
Another economic factor is the cost of raising and educating children. Birth and fertility rates tend to be lower in more-developed countries, where raising children is much more costly because they do not enter the labor force until they are in their late teens or twenties. In the United States, the U.S. Department of Agriculture estimated that the average cost of raising a child born in the United States in 2018 to the age 18 was nearly $234,000.
The availability of pension systems can influence the number of children couples have, especially poor people in less-developed countries. Pensions reduce a couple’s need to have several children to replace those that die at an early age and to help support them in old age.
Urbanization also plays a role. People living in urban areas usually have better access to family planning services and tend to have fewer children than do those living in the rural areas of less-developed countries.
Another important factor is the educational and employment opportunities available for women. Total fertility rates tend to be low when women have access to education and paid employment outside the home. In less-developed countries, a woman with no education typically has two more children than does a woman with a high school education.
The average age at which a woman has her first child also plays a role. Women normally have fewer children when their average age at their first child’s birth is 25 or older.
Birth rates and TFRs are also affected by the availability of reliable birth control methods that allow women to control the number and spacing of their children.
Religious beliefs, traditions, and cultural norms also play a role. In some countries, these factors contribute to large families, because many people strongly oppose abortion and some forms of birth control.
Learning from Nature
Anthropologists have long been interested in how isolated populations of indigenous people have controlled population growth for centuries, even where environmental conditions favor population growth. Cultural factors, mostly related to long-established marriage practices, have been found to act as natural birth control measures.
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6.2dFactors That Affect Death Rates
The rapid growth of the world’s population over the past
100
years is largely the result of declining death rates, especially in less-developed countries. More people in some of these countries live longer, and fewer infants die because of larger food supplies, improvements in food distribution, better nutrition, improved sanitation, safer water supplies, and medical advances such as immunizations and antibiotics.
A useful indicator of the overall health of people in a country or region is
life expectancy
: the average number of years a person born in a particular year can be expected to live. Between 1955 and 2018, average global life expectancy increased from 48 years to 72 years. Between 1900 and 2018, the average U.S. life expectancy rose from
47
years to 79 years. Research indicates that poverty, which reduces the average life span by 7 to 10 years, is the single most important factor affecting life expectancy. For example, the average life expectancy in the world’s 10 poorest nations is 55 years compared to 80 years in the 10 wealthiest nations.
Another important indicator of the overall health of a population is its
infant mortality rate
, the number of babies out of every 1,000 born who die before their first birthday. It is viewed as one of the best measures of a society’s quality of life because it indicates the general level of nutrition and health care. A high infant mortality rate usually indicates insufficient food (undernutrition), poor nutrition (malnutrition; see
Figure 1.14
), and a high incidence of infectious disease. Infant mortality also affects the TFR. In areas with low infant mortality rates, women tend to have fewer children because fewer of their children die at an early age.
Infant mortality rates in most countries have declined dramatically since 1965 (
Figure 6.9
). Even so, every year more than 4 million infants die of preventable causes during their first year of life, according to UN population experts. Most of these deaths occur in less-developed countries. This average of nearly 11,000 mostly unnecessary infant deaths per day is equivalent to 55 jet airliners, each loaded with 200 infants, crashing every day with no survivors.
Figure 6.9
Comparison of infant mortality rates in more-developed countries and less-developed countries, 1950–2018, with projections to 2050 based on medium population projections.
(Compiled by the authors using data from United Nations Population Division and Population Reference Bureau.)
Between 1900 and 2018, the U.S. infant mortality rate dropped from 165 to 5.6. This sharp decline was a major factor in the marked increase in U.S. average life expectancy during this period. However, 53 other nations had lower infant mortality rates than the United States in 2018.
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6.2eMigration
A third factor in population change is
migration
: the movement of people into (immigration) and out of (emigration) specific geographic areas. Most people who migrate to another area within their country or to another country are seeking jobs and economic improvement. Others are driven by religious persecution, ethnic conflicts, political oppression, or war. There are also environmental refugees—people who have to leave their homes and sometimes their countries because of water or food shortages, soil erosion, or some other form of environmental degradation.
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6.3aAge Structure
The age structure of a population is the numbers or percentages of males and females in young, middle, and older age groups in that population. Age structure is an important factor in determining total fertility rates and whether the population of a country increases or decreases.
Population experts construct a population age-structure diagram by plotting the percentages or numbers of males and females in the total population in each of three age categories: pre-reproductive (ages 0–14), consisting of individuals normally too young to have children; reproductive (ages 15–44), those normally able to have children; and post-reproductive (ages
45
and older), with individuals normally too old to have children.
Figure 6.10
presents generalized age-structure diagrams for countries with rapid, slow, zero, and negative (declining) population growth rates.
Figure 6.10
Generalized population age-structure diagrams for countries with rapid (1.5–3%), slow (0.3–1.4%), zero (0–0.2%), and negative (declining) population growth rates.
Question:
1. Which of these diagrams best represents the country where you live?
(Compiled by the authors using data from U.S. Census Bureau and Population Reference Bureau.)
A country with a large percentage of people younger than age 15 (represented by a wide base in
Figure 6.10, far left) will experience rapid population growth unless death rates rise sharply. Because of this demographic momentum, the number of births in such a country will rise for several decades. This will occur even if women each have an average of only one or two children because of the large number of girls entering their prime reproductive years. Most future human population growth will take place in less-developed countries because of their typically youthful age structure and rapid population growth rates.
300%
Projected increase in the global population of people over 65 between 2016 and 2050
The global population of seniors—people who are 65 and older—is projected to triple between 2018 and 2050 when one of every six people will be a senior. (See the Case Study that follows.) An aging population combined with a lower fertility rate results in fewer working-age adults having to support a large number of seniors. For example, in China and the United States between 2010 and 2050, the working-age population is projected to decline sharply. This could lead to a shortage of workers and friction between the younger and older generations in these countries.
Case Study
The American Baby Boom
Changes in the distribution of a country’s age groups have long-lasting economic and social impacts. For example, the American baby boom (Figure 6.5) added 79 million people to the U.S. population between 1946 and 1964. Over time, this group looks like a bulge as it moves up through the country’s age structure, as shown in
Figure 6.11
.
Figure 6.11
Age-structure diagrams tracking the baby-boom generation in the United States, 1955, 1985, 2015, and 2035 (projected).
Critical Thinking:
1. How might the projected age structure in 2035 affect you?
(Compiled by the authors using data from U.S. Census Bureau and Population Reference Bureau.)
For decades, the baby-boom generation has strongly influenced the U.S. economy because it makes up about 25% of the U.S. population. Baby boomers created the youth market in their teens and twenties and are now creating the late middle age (ages 50 to 60) and senior markets. In addition to having this economic impact, the large baby-boom generation plays an important role in deciding who is elected to public office and what laws are passed or weakened.
Since 2011, when the first baby boomers began turning 65, the number of Americans older than age 65 has grown at the rate of about 10,000 a day and will do so through 2030. This process has been called the graying of America. As the number of working adults declines in proportion to the number of seniors, there may be political pressure from baby boomers to increase tax revenues to help support the growing senior population. However, in 2015, the Millennial Generation—Americans born between 1980 and 2005—overtook Baby Boomers to become the largest generation living in the United States. This could lead to economic and political conflicts between older and younger Americans.
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6.3bAging Populations Can Decline Rapidly
The graying of the world’s population is due largely to declining birth rates and medical advances that have extended life spans. The UN estimates that by 2050, the global number of people age 60 and older will equal or exceed the number of people under age 15 (
Figure 6.12
).
Figure 6.12
The world’s age structure 1950, 2010, and 2050 (projected).
Critical Thinking:
1. How might the projected age structure in 2050 affect you?
(Compiled by the authors using data from U.S. Census Bureau and United Nations Population Division.)
As the percentage of people age 65 or older increases, more countries will begin experiencing population declines. If population decline is gradual, its harmful effects usually can be managed. However, some countries, such as Japan, are experiencing rapid declines and feeling such effects more severely.
Japan has the world’s highest percentage of people age 65 or over and the world’s lowest percentage of people under age 15. In 2018, Japan’s TFR was 1.4 births per woman, one of the lowest in the world. In 2018, Japan’s population was 127 million. By 2050, its population is projected to be 102 million. As its population declines, there will be fewer adults working and paying taxes to support an increasingly elderly population. Because Japan discourages immigration, this could threaten its economic future. In recent years, Japan has been feeling the effects of a declining population. For example, houses in some suburbs have been abandoned and cannot be sold because of a lack of buyers. They could be demolished, but who will pay the costs—the owners who have abandoned them, or the government?
Figure 6.13
lists some of the problems associated with rapid population decline. Population declines are difficult to reverse
Figure 6.13
Rapid population decline can cause several problems.
Critical Thinking:
1. Which two of these problems do you think are the most important?
Top: Slavoljub Pantelic/ Shutterstock.com. Center: Iofoto/ Shutterstock.com. Bottom:sunabesyou/ Shutterstock.com.
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6.4aEconomic Development
Some analysts argue that we need to slow population growth in order to reduce degradation of our life-support system. They have suggested several ways to do this, one of which is to reduce poverty through economic development.
Demographers have examined the birth and death rates of western European countries that became industrialized during the 19th century. Using such data, they developed a hypothesis on population change known as the
demographic transition
. It states that as countries become industrialized and economically developed, their per capita incomes rise, poverty declines, and their populations tend to grow more slowly. According to the hypothesis, this transition takes place in four stages, as shown in
Figure 6.14
. Some good news for those who view population growth as a serious environmental problem is that by 2018, 38 countries, mostly in Europe, had stabilized their populations or were experiencing population declines.
Figure 6.14
The demographic transition, which a country can experience as it becomes industrialized and more economically developed, can take place in four stages.
Question:
1. At what stage is the country where you live?
Some analysts believe that most of the world’s less-developed countries will make a demographic transition over the next few decades. They hypothesize that the transition will occur because newer technologies will help them to develop economically and to reduce poverty.
Other analysts fear that rapid population growth, extreme poverty, war, increasing environmental degradation, and resource depletion could leave some countries with high population growth rates stuck in stage 2 of the demographic transition. This highlights the need to reduce poverty as a key to improving human health and stabilizing population.
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6.4bEducating and Empowering Women
A number of studies show that women tend to have fewer children if they are educated, can control their own fertility, earn an income of their own, and live in societies that do not suppress their rights. In most societies, women have fewer rights and fewer educational and economic opportunities than men have.
Women do almost all of the world’s domestic work and childcare for little or no pay. They provide more unpaid health care (within their families) than do all of the world’s organized health-care services combined. In rural areas of Africa, Latin America, and Asia, women do 60–80% of the work associated with growing food, hauling water, and gathering and hauling wood (
Figure 6.15
) and animal dung for use as fuel. As one Brazilian woman observed, “For poor women, the only holiday is when you are asleep.”
Figure 6.15
This woman in Nepal is bringing home firewood. Typically, she spends 2 hours a day, two or three times a week, on this task.
Iv Nikolny/ Shutterstock.com
Globally, women spend 90% of their income on their immediate family needs, and own just 10% to 20% of the world’s land. Women of childbearing age make up 55% of the poor, and more than two-thirds of the world’s 7
75
million illiterate adults are women. Poor women who cannot read often have an average of five to seven children, compared with two or fewer children in societies where most women can read. This highlights the need for all children to get at least an elementary school education. In addition, if the survival rates of children can be raised, parents will be able to have fewer children and feel confident that most of their children will survive to adulthood.
A growing number of women in less-developed countries are taking charge of their lives and reproductive behavior. As this number grows, such change driven by individual women will play an important role in stabilizing populations. This change will also improve human health and reduce poverty and environmental degradation.
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6.4cFamily Planning
Family planning
programs provide education and clinical services that can help couples to choose how many children to have and when to have them. Such programs vary from culture to culture, but most of them provide information on birth spacing, birth control, and health care for pregnant women and infants.
According to studies by the UN Population Division and other population agencies, family planning has been a major factor in reducing the number of unintended pregnancies, births, and abortions. In addition, family planning has reduced rates of infant mortality, the number of mothers and fetuses dying during pregnancy, and population growth rates. According to the UN, had there not been the sharp drop in TFRs since the 1970s, with all else being equal, the world’s population today would be about 8.5 billion instead of 7.6 billion (
Core Case Study
). Family planning has played an important role in countries that have stabilized their populations.
Family planning also has financial benefits. Studies show that each dollar spent on family planning in countries such as Thailand, Egypt, and Bangladesh saves $10 to $16 in health, education, and social service costs by preventing unwanted births.
Despite these successes, certain problems have hindered progress in some countries. There are three major problems. First, according to the UN Population Fund and the Guttmacher Institute, about 40% of all pregnancies in less-developed countries were unplanned and about half of these pregnancies end with abortion. So, ensuring access to voluntary contraception would play a key role in stabilizing the populations and reducing the number of abortions in such countries.
Second, according to the UN Population Fund, an estimated 214 million women, primarily in the world’s poorest countries, are not using safe and effective family planning methods. Meeting these current unmet needs for family planning and contraception could prevent more than 80 million unintended pregnancies, 36 million induced abortions (millions of them unsafe), 1 million infant deaths, and 76,000 pregnancy-related deaths of women per year. It would also provide economic benefits. For every dollar invested in contraception, the cost of pregnancy-related care is reduced by $2.20.
Third, largely because of cultural traditions, male domination, and poverty, one in every three girls in less-developed countries is married before age 18 and one in nine is married before age 14. This occurs despite laws against child marriage. For a poor family, marrying off a daughter can relieve financial pressure.
Some analysts call for expanding family planning programs to educate men about the importance of having fewer children and taking more responsibility for raising them. Proponents also call for greatly increased research in order to develop more effective birth control methods for men.
The experiences of countries such as Japan, Thailand, Bangladesh, South Korea, Taiwan, and China show that a country can achieve or come close to replacement-level fertility within a decade or two. The real population story of the past 50 years has been the sharp reduction in the rate of population growth (
Figure 6.2) resulting from the reduction of poverty through economic development, empowerment of women, and the promotion of family planning. However, the global population is still growing fast enough to add up to 3 billion more people by 2050.
Case Study
Population Growth in India
For more than six decades, India has tried to control its population growth with only modest success. The world’s first national family planning program began in India in 1952, when its population was nearly 400 million. In 2018, after 66 years of population control efforts, India had 1.4 billion people—the world’s second largest population and a TFR of 2.3. Much of this increase occurred because of India’s declining death rates.
Three factors help to account for larger families in India. First, most poor couples believe they need several children to work and care for them in their old age. Second, the strong cultural preference in India for male children means that some couples keep having children until they produce one or more boys. Third, although 90% of Indian couples have access to at least one modern birth control method, only about 48% actually used one in 2018, according to the Population Reference Bureau.
Figure 6.16
shows changes in India’s age structure between 2010 and 2035 (projected). The UN projects that by 2029, India will be the world’s most populous country, and that by 2050, it will have a projected population of 1.7 billion.
Figure 6.16
Age structure changes in India: 2010 and 2035 (projected).
Critical Thinking:
1. How might the projected age structure in 2035 affect India’s ability to reduce poverty?
(Compiled by the authors using data from U.S. Census Bureau and United Nations Population Division.)
India has the world’s fourth largest economy and a rapidly growing middle class. However, the country faces serious poverty, malnutrition, and environmental problems that could worsen as its population continues to grow rapidly. India is home to one-third of the world’s poorest people. About one-fourth of all people in India’s cities live in slums, and prosperity and progress have not touched hundreds of millions of Indians who live in rural villages. According to the World Bank, about 30% of India’s population—one-third of the world’s extremely poor people—live in extreme poverty on less than $1.25 per day (
Figure 6.17
). For decades, the Indian government has provided family planning services throughout the country and has strongly promoted a smaller average family size. Even so, Indian women have an average of 2.3 children.
Figure 6.17
Homeless people in Kolkata, India.
Samrat35/ Dreamstime.com
India also faces critical resource and environmental problems. With 18% of the world’s people, India has just 2.3% of the world’s land resources and 2% of its forests. About half the country’s cropland has been degraded by soil erosion and overgrazing. In addition, more than two-thirds of its water is seriously polluted, sanitation services often are inadequate, and many of its major cities suffer from serious air pollution.
India’s rapid economic growth is expected to accelerate over the next few decades. This will help many people in India escape poverty, but it will also increase pressure on India’s and the earth’s natural capital as per capita resource use increases. India already faces serious soil erosion, overgrazing, water pollution, and air pollution problems. On the other hand, economic growth may help India to slow its population growth by accelerating its demographic transition (
Figure 6.14).
Case Study
Slowing Population Growth in China
China is the world’s most populous country, with 1.39 billion people in 2018. According to the Population Reference Bureau and the United Nations Population Fund, China’s population is projected to peak at 1.4 billion in 2030 and then to decline to 1.3 billion by 2050.
In the 1960s, China’s large population was growing so rapidly that there was a serious threat of mass starvation. To avoid this, government officials took measures that eventually led to the establishment of the world’s most extensive, intrusive, and strict family planning and birth control program.
The goal of the program, established in 1978, has been to sharply reduce population growth by promoting one-child families. The government provided contraceptives, sterilizations, and abortions for married couples. Married couples pledging to have no more than one child received better housing, more food, free health care, salary bonuses, and preferential job opportunities for their child. Couples who broke their pledge lost such benefits.
Since this government-controlled program began in 1978, China has made impressive efforts to feed its people and bring its population growth under control. Between 1972 and 2018, the country reduced its TFR from 3.0 to 1.8—one of the fastest demographic transitions (Figure 6.14) in history. China’s population is now growing more slowly than the U.S. population. Although China has avoided mass starvation, its strict population control program has been accused of violating human rights.
An unintended result of China’s population control program is that because of the cultural preference for sons, many Chinese women have aborted female fetuses. This has reduced the female population and it is estimated that by 2020, there will be 30 million more Chinese men than women looking for a partner.
Since 1980, China has undergone rapid industrialization and economic growth. According to the Earth Policy Institute, between 1990 and 2010 this process reduced the number of people living in extreme poverty by almost 500 million. It has also helped more than 400 million Chinese—a number greater than the entire U.S. population—to become middle-class consumers. However, millions of people live in poverty in China’s villages and cities (
Figure 6.18
). China’s rapidly growing middle class will consume more total resources. This will put a strain on China’s and the earth’s natural capital. Like India, China faces serious soil erosion, overgrazing, water pollution, and air pollution problems.
Figure 6.18
Old and new housing in heavily populated Shanghai, China, in 2015.
Nikada/iStock/Getty Images Plus/Getty Images
Because of its one-child policy, in recent years the average age of China’s population has been increasing at one of the fastest rates ever recorded. In 2018, at least 123 million Chinese people were over age 65—the largest number of people in this age group of all the world’s countries.
Figure 6.19
shows China’s age structure in 2010 and its projected age structure in 2035. Since 2017, China’s birth rate has declined, dropping by 12% in 2018. The UN estimates that by 2030, the country is likely to have too few young workers (ages 15 to 64) to support its rapidly aging population. This graying of the Chinese population could lead to a declining work force, limited funds for supporting continued economic development, and fewer children and grandchildren to care for the growing number of elderly people. These concerns and other factors may slow China’s economic growth. To help deal with this problem, China plans to become the world’s largest manufacturer of industrial robots to be used for manufacturing. It will also sell such robots to other countries.
Figure 6.19
Age structure in China: 2010 and 2035 (projected).
Critical Thinking:
1. How might the projected age structure in 2035 affect China’s economy?
(Compiled by the authors using data from U.S. Census Bureau and United Nations Population Division.)
Because of these concerns, in 2015, the Chinese government abandoned its one-child policy and replaced it with a two-child policy. Married couples can apply to the government for permission to have two children. However, because of the high cost of raising a second child, and because young women enjoy greatly increased educational and job opportunities, many married couples still choose to have only one child.
Big Ideas
· The human population is growing rapidly and may soon bump up against environmental limits.
· The combination of population growth and the increasing rate of resource use per person is expanding the overall human ecological footprint and putting a strain on the earth’s natural capital.
· We can slow human population growth by reducing poverty, elevating the status of women, and encouraging family planning.
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Tying It All TogetherWorld Population Growth and Sustainability
Jeremy Richards/ Shutterstock.com
This chapter began with a discussion of the fact that the world’s human population has now reached 7.6 billion (
Core Case Study). We noted that this is a result of exponential population growth and that many environmental scientists believe such growth to be unsustainable in the end. We briefly considered some of the environmental problems brought on by exponential human population growth. We looked at factors that influence the growth of populations, as well as at how some countries have made progress in controlling population growth.
In the first six chapters of this book, you have learned how ecosystems and species have been sustained throughout the earth’s history, in keeping with the three scientific principles of sustainability, by nature’s reliance on solar energy, nutrient cycling, and biodiversity (see inside back cover of this book). These three principles can guide us in dealing with the problems brought on by population growth and decline. By greatly increasing our use of solar, wind, and other renewable-energy technologies, we can cut pollution and emissions of climate-changing gases that are increasing as the population and resource use per person grow. By reusing and recycling more materials, we can cut resource waste and reduce our ecological footprints. By focusing on preserving biodiversity, we can help sustain the life-support system on which we and all other species and our economies depend.
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Chapter Review
Chapter Review
Critical Thinking
1. Do you think that the global population of 7.6 billion (
Core Case Study) is too large? Explain. If your answer was yes, what do you think should be done to slow human population growth? If your answer was no, do you believe that there is a population size that would be too big? Explain.
2. If you could say hello to a new person every second without taking a break and working around the clock, how many people could you greet in a day? How many in a year? How long would it take you to greet the 91 million people who were added to the world’s population in 2018? At this same rate, how many years would it take you to greet all 7.7 billion people living on the earth in 2018?
3. Of the three major environmental worldviews summarized in
chapter 1
, which do you think underlies each of the two major positions on whether the world is overpopulated, as described in Science Focus 6.1? Explain.
4. Should everyone have the right to have as many children as they want? Explain. Is your belief on this issue consistent with your environmental worldview?
5. Is it rational for a poor couple in a less-developed country such as India to have four or five children? Why or why not?
6.
Do you think that projected increases in the earth’s population size and economic growth are sustainable? Explain. If not, how is this likely to affect your life?
7. Some people think the most important environmental goal is to sharply reduce the rate of population growth in less-developed countries, where at least 95% of the world’s population growth is expected to take place between 2018 and 2050. Others argue that the most serious environmental problems stem from high levels of resource consumption per person in more-developed countries. What is your view on this issue? Explain.
8. Experts have identified population growth as one of the major causes of the environmental problems we face. The population of the United States is growing faster than that of any other more-developed country. This fact is rarely discussed and the U.S. government has no official policy for slowing U.S. population growth. Why do think this is so? Do you think there should be such a policy? If so, explain your thinking and list three steps you would take as a leader to slow U.S. population growth. If not, explain your thinking.
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Chapter Review
Doing Environmental Science
1. Prepare an age-structure diagram for your community. You will need to estimate how many people belong in each age category (see Figure 6.11). To do this, interview a randomly drawn sample of the population to find out their ages and then divide your sample into age groups. (Be sure to interview equal numbers of males and females.) Then find out the total population of your community and apply the percentages for each age group from your sample to the whole population in order to make your estimates. Create your diagram and then use it to project future population trends. Write a report in which you discuss some economic, social, and environmental effects that might result from these trends.
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Chapter Review
Data Analysis
The
chart
below shows selected population data for two different countries, A and B. Study the chart and answer the questions that follow.
|
Country A |
Country B |
|
|
Population (millions) |
144 |
82 |
|
Crude birth rate (number of live births per 1,000 people per year) |
43 |
8 |
|
Crude death rate (number of deaths per 1,000 people per year) |
18 | 10 |
|
Infant mortality rate (number of babies per 1,000 born who die in first year of life) |
100 |
3.8 |
|
Total fertility rate (average number of children born to women during their childbearing years) |
5.9 |
1.3 |
|
% of population under 15 years old |
45 | 14 |
|
% of population older than 65 years |
3 | 19 |
|
Average life expectancy at birth |
47 | 79 |
|
% urban |
44 | 75 |
1. Calculate the rates of natural increase (due to births and deaths, not counting immigration) for the populations of country A and country B. Based on these calculations and the data in the table, for each of the countries, suggest whether it is a more-developed country or a less-developed country and explain the reasons for your answers.
2. Describe where each of the two countries might be in the stages of demographic transition (Figure 6.14). Discuss factors that could hinder either country from progressing to later stages in the demographic transition.
3. Explain how the percentages of people under age 15 in each country could affect its per capita and total ecological footprints.
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Chapter Introduction
Slum area (bottom) in Mumbai, India
ZUMA Press, Inc./Alamy Stock Photo
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Core Case Study
Planet Earth:
Population 7.6 Billion
Learning Objectives
·
LO 6.1
Describe the
growth of the human population in terms of numbers
of years between doublings.
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Chapter Introduction
Slum area (bottom) in Mumbai, India
ZUMA Press, Inc./Alamy Stock Photo
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Core Case StudyPlanet Earth:
Population 7.6 Billion
Learning Objectives
LO 6.1Describe the growth of the human population in terms of numbers
of years between doublings.
Livingin the Environment (MindTap Course List)
20th Edition
ISBN-13: 978-0357142202, ISBN-10: 0170291502
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